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Mr. N.'s well-digested Meteorological Table for Hull has been re- ceived ; but were we to give general admission to such tables, they would occupy the whole of our pages.

Dr. Sandis's Experiments in onr next, if possible.

Mr. Murray's interesting Communications on Chlorine and Chlorate of iPotash ; On the Relation of Acids and Alkalis to vegetable Colours ; On Air taken from an Ice-house, and On Urinary Calculi, will appear as speedily as ive can make room for them.

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T. G.'s Communication vespecting a Meteor seen in 1814 refers to an occurrence rather too remote for notice in the year 1822.

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' ' ' By JAMES PARKINSON,

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TABLES to be used with the NAUTICAL ALMANAC, for finding the LATITUDE and LONGITUDE at SEA.

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Cambridge. Containing a complete and compendious Set-of Tables, in which the quantities are taken out by immediate inspection, and the proportional parts are additive ; with easy and accurate methods of solving the various Problems required, and the newest determinations of the Latitudes and Longitudes of places: intended as a Substitute for the Requisite Tables.

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Vol. 60.

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No. 292.

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Mr. RiDDtE's Letter, and Mr. D. Mushet on the Origin and Discovery of Iron, in our next.

Mr. Saml-ei. Ta-vi-or's Communication on the Causes of a singular partial Failure in the Growth of Turnips, is received. We have also to thank him for Obser\ations on the French Report on the early Cutting of Wheat in our present Number.

We have not room to continue the Phrenological Controversy.

The great number of characters which typography does not furnish has been the obstacle to our hitherto availing ourselves of Mr. Upington's continuation.

Mr. P. Nicholson on Equations, Di*. De Sanctis's Experiments, Mr. Gut- TERiDGE on Weights and Measures, Mr. R. H. Goaver's Description of a Life Boat, and Mr. Meikle on the Lunar Distance, will appear as soon as possible.

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TABLES to be used with the NAUTICAL ALMANAC, for finding the LATITUDE and LONGITUDE at SEA.

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Sept. 1822.

No. 293.

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hllustrative of ^ Paper by F. Baily, Esq. on the Stars forming . the Pleiades.

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Tlie Letter from Professor Littrow to Baron Zach has been received; also a second Communication on Iron from Mr. D. Musiiet ; and a Letter froiri . Mr. W. DoBBiE.

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No. 295.

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For N 0 FE M B E R 1822.

By ALEXANDER TILLOCH, LL.D.

M.R.I. A. M.O.S. M.A.S. F.S.A. EDIN. AND PERTH ; CO KRE3P0NDI NO MRM-J DER OF THE ROYAL ACADEMY OF SCIENCES, MUNICH ; AND OF THe[ ACADEMY OP SCIENCES, LITERATURE AND ARTS, lEGHORNi hi. &,C. ^^^

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The ri)llo\\ ing Coiiiimmicatioiu have been received :

On AeriiU Navigation. Mr. Gutteridge on the Measurement of Timber.— On Pillar-work and Way-going-work in Collieries. Mr.MusHET on the Origin of the Blast Furnace.

We must decline inserting E's Letter on the controversy respecting Mr. Hera- path's Theory, as it treats only on personal topics of which, no doubt, our readers will tliink they have already had too much. We agree vi-ith E, " that the scientific public have only to do with the Theory itself, and not with individual dispositions and feelings;" and we do not wish to have a controversy prolonged, when it has assumed this character.

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CON T E N T S OF THE SIXTIETH VOLUME.

]FuBTHER Observations on Dr. Reade's Papers onHefraction

5

An Account of the Repeating Circle, and of the Altitude and Azimuth Instrument 8, 102

On the Hypothesis of Gaseous Repidsion 18

On the Solar Eclipse vohich took place on September 7> 1820 25

Account of a successful Experime^it to prevent the injuriotis

Effects resulting from the Diffusion of arsenical Vapours from

Copper-smelting Furnaces 32

On the Connexion between the Leaves and Fruit of Vegetables, Sfc 38

Fw'ther Remarks on the new Method of determining the Lati- tude of a Place, by Observations of the Pole-Star . . 43

True aj)parent Right Ascension of Dr. Maskelyne's 36 Stars f(/r evety Day in the Year 1822, at the Time of passing the Meridian of Greenwich 49, 93

On the Pollen of Flowers \ 56

On the Heat produced by Chlorine, and on a singidar Effect produced by Lightning 61

A Reply to M?-. John Murray, "on the Apparatus for restor- ing the Action of the Ltmgs iti apparent Death " . , 62

On the Chemical Composition of white efflorescent Pyrites 6.5

On the Hygrojneter by Evaporation 81

On a new Theory of the Tides 88

On Chlcnine and Chlorate of Potassa 100

Jteport of a Committee of the House of Commons on Steam-Boats ; being the Fifth Report on the Roads from London to Holy- hcad,Sfc 113,257

Vol. 60. 1822, ii

CONTENTS.

Fossil Bones on the Coast of East Norfolk 132

On the Origin and Discovei'y of Iron 161, 249

Suggestions for simplifijing Mr. Ivory's Solution of the Double Altilude'Prohlcm 167

On the Bclation of Acids and Alkalis to vegetable Colours^ and their Mutations thereby 170

'Ne'jn Demonstrations of the Method invented by Budan, and improved by others, of extracting the Roots of liquations 173

On the Electrical Phcenomena exhibited in Vacuo . . . 179

On M}\ John MooR^ Ju?iior's " Reply" 186

On the Causes of a singular partial Failure in a Crop of Tur- nips 187

On the Stars forming the Pleiades 189

Reply to Dr. Apjohn's " Additional Remarks," t5T. i7i The Annals oiFhWohophy for September 1822 .... 193

On simidtaneous Thunder-Storms 195

Remarks respecting Astronomical Observatories . . . 197 A Letter to Professor Mii.lington, of the Roycd Insiitution, re- specting some Frigori/ic Experiments made on the Magnetic

Fluid, a7id on Sea-Water 199

On Lithographic Printing 206

Short Account of the Rocks in the Neighbourhood of St. Johfi's, Newfoundland ,. 206

Rejjly to Captain FoRMAi^^s Theory of the Tides . . . 211

On the piroposed Alteration of Weights and Measures, and the

Adaptation of Gaugi7ig Apparatus thereto .... 241

On an Lnprovemcnt in the Apparatus for procuring Potassiuin

247 On the Application of Magnetism as a Measure of Electricity 253

On the Repeating Circle improved according to the Suggestion of Baron Zach : and on taking Observations of the Polc- Star 263

Analysis of Air taken from an Ice-House 266

A Defence of the new Theory of the Tides 267

On reducing the Lunar Distance 271

On some new Tables of Aberration and Nutation . . . 279

DJs Second Reply to C. pn Mr. HEUArATn's Theory . 285

On a nexv Sextant recently invented a7id constructed by Pro- fessor Amici 301

CONTENTS.

Observations on the occasional Appearmice of Water in the Ca- vities of regular Crystals; a7id on the porous Nature ofQiutrtz and other Crystalline Substances, as the probable Cause of thai

Occurreiice 31ft

On a neio circular Micrometer 314<

Mr. P. Taylor on Pyroligneous JEther 315

On a Lunar Iris, or Rainbow by Moonlight . . . , 317

Note on Mr. Murray's Paper on the Relation of Acids and Alkalies to Vegetable Colours 319

On the Measurement of the Progress of an Eclipse of the Moon

•with a Sextant or Reflecting Circle 329

Reply to Captain Forman on the Theoiy of the Tides . 335 Observations on the Flexure of Astronomical Instruments . 338 On Governor Ellis's Discovery oftheAcfion of Cold on Mag- netic Needles 340

Description of a neia Printing Press 341

Autumnal Blowing of the Narcissus 343

Oti the anomalous magnetic Action of hot Iron between the white and blood-red Pleat 343

On Sviut ill JVhcat 35O

DescrijJtion o/" Hemipodius nivosus; a new Bird from Africa.

S53

A Letter from John Pond, Esq. Astronomer Royal, to Sir Humphry Davy, Barf. President of the Royal Society, rela- tive to a Derangement in the Mural Circle at the Royal Ob- servatory 355

On the Subside7ice of aqueous Vapow, and its RcjJOse on the Beds of Rivers 357

Notice of some new Galvanic Exp)eriments and Phanomeria 358 On the North Polar Distafices of the principal fixed Stars, de- duced from the Observations made at the Royal Observatory at Greenwich 361

Experiments on the Alloys of Steel, made with a view to its Ln-

provement *355, 363

Mr. Moore on Mr. J. Murray's Communications . . 374

Inquiry resp)ecting Floods in Dorsetshire 377

On the Origin of the Blast Furnace 401

Description of a Life-Boat, built by Subscriptioti at Ipsxvichy

and stationed at Landguard Fort 409

On Ihc Mcauiremcnl of Timber lis

CONTENTS.

On the Theory of parallel Lines in Geometry . , . . 423 On the Autumnal Flowering of the Narcissus .... 426 An Account of a General Survey of the Heavens undertaken at

the Konigsberg Observatory 427

Some Experiments and Researches on the Saline Contetits of Sea- Water, undeHaken "with a view to correct and improve its Che- mical Analysis 434

On the visible Solar and Lunar Eclipses which will happen in

the Year 1823 . 440

Some Remarks on Urinary Calculi, and their Chemical Exami- nation 445

Note of some Experiments on the Vapour ofSidphuric Ether 448

Tnie apparent Right Ascension of Dr. Maskelyne's 36 Stars

for every Day in the Year 1823, at the Time cf passing the

Meridian of Gree?iwich 449

Mr. P. Taylor on the Expansive Farce of Steam at diffei'ent Temperatures 452

Notices respecting New Books . . 66, 1 35, 21 1, 3 1 9, 377, 454 Proceedings of Learned Societies . 67, 145, 229, 387, 459 Intelligeyice and Miscellaneous Articles 66, 147, 230, 321, 388,

467 List of Patents 79,159,239,326,399,470

Meteorological Tables . . . . 80,160,240,328,400,472

PLATES.

I. Pollen of Flowers 5(5

II. Fossil Bones from Norfolk . . . . 13j2

III. Pleiades 189

IV. Amici's Sextant 301

V. Gower'6 Life-Boat 409

VI. Taylor's Table of Expansion of Steam 452

THE

PREFACE.

It is now within a few months of a quarter of a cen- tury since this work wa^ commenced ; a period rich in scientific discoveries, the records of which will be referred to with deep interest by future generations.

During this long period lio exertion has been wanting on my part to render The Philosophical Magazine and Journal worthy of the flattering re- ception it has experienced, not only at home, but throughout the civilized world. That the work is a perfect one, it would ill become me to assert ; but I believe that, without vanity, I may say, that with all its defects, whatever these may be, it has tended not a little to diffuse a love of science and the liberal arts among the present generation.

1 beg to return unfeigned thanks to my numerous friends for the aids they have afforded mc in conduct- ing the work, and rendering my miscellany really use- ful to the world; and have now to announce, that to render this work still more worthy of the patronage

IV PREFACE.

it receives from the public, I have obtained the co- operation of Mr. Richard Taylor, a gentleman from whose exertions, in conjunction vv^ith my own, and none I am sure will be wanting, I may, with- out presumption, hope that The Philosophical Magazine and Journal will increase in interest and general utility.

ALEX. TILLOCH.

London, July 31, ISSiJ.

(fj* Communications for the Philosophical Magazine and Journal are requested to be addressed in future to the Editors, care of R. and A. TAyi,OR, Shoe-Lane.

THE

PHILOSOPHICAL MAGAZINE AND JOURNAL.

JULY 1822.

I. Furthe)' Observations on Dr. Reade's Papers on Refraction^ By Mr. Charles Stark, R.N.

To Dr. Tilloch.

Sir, jL our correspondent Dr. Reade having, in the Num- ber for Marcli, favoured us with another paper ilkistrative of his new System of Optics, I beg to offer a few more observa- tions on that interesting subject.

From the Doctor's former paper, it appeared that he felt quite convinced of having demonstrated that no such thing existed as the refraction of hght ; and that what philosophers had hitherto attempted to explain by having recourse to that principle, might be accounted for much more simply on the principle of reflection. It appears too, from his last commu- nication, that his opinions on this subject still remain un- shaken,— that he feels quite convinced of having completely subverted the Newtonian system of optics : besides, he tells us that his opinions are daily gaining ground, and received by men of the first eminence.

In entering into any critical examination of Dr. R.'s papers, I have no wish to throw a shade over the bright j^rospects which he must, no doubt, be enjoying of seeing that his name must stand pre-eminent in the future annals of philosophical discovery; nor am I actuated by any "angry" motives, as he has unfortunately been led to suj)pose ; but with every feeling of respect to Dr. Reade as a gentleman and a man of science, I shall here take the liberty to ]X)int out to him and his fol- lowers, the absolute necessity of admitting the law of refrac- tion as well as reflection into the science of optics.

Dr. Reade, at the commencement of his last paper, again Vo 60. No. 291. ,/?//?/ 1822. gives

6 Ohservatiotn on Hefraclion.

gives a detail of the experiment of placing a piece of money at the bottom of a tumbler partly filled with water ; and from his observing that the piece was seen when the tumbler was held below, on a level with, and above the eye, he concludes that in each case it is seen by a reflected image formed perpendi- cularly over it on the sm-face of the water.

Before entering into any examination of the weight of Dr. R.'s arguments, I shall here mention two experiments which I think are of themselves quite sufficient to set at rest the whole of his rea-soning on the subject.

Exp. 1. To jireclude the possibility of the surface of the water in the tunibler becoming a reflecting surface, I covered it over with a circular piece of dry flannel, with a small semi- circular hole cut out of its edge. On holding the tumbler be- low the level of the eye, the half-crown was seen through the opening in the edge, in the same manner as before it was co- vered ; but on holding it above the eye no image whatever coukl be seen. I would here ask Dr. K., how it happens that the reflected image is destroyed in the one case and not in the other.

Exp. 2. Having formed a small tube ^.^^

of pasteboard with an angular bend in it [ 'd^^::^^^^^\

at D, as in the figure, so that nothing - ^^y^- b

could be seen through it in the open air, /'

I placed the end C on the bottom of the c

tumbler A B, the part C D making an

oblique angle with the surface of tlie water, and having tlie point D exactly in that surface. By holding a candle under the point C, and looking through the tube from the other end E, the bottom of the tumbler was seen quite distinctly, the tube at the same time appearing nearly straight. When a straight tube was used and held in the same direction C D, nothing whatever could be seen through it. From this ex- periment I may also draw Dr. Reade's weighty inference, that " ifo see is to believe " but that to see an^ object through a bended tube in the above manner, is to believe that the rays of light in their progress from the object to the eye follow the direction and bend of that tube ; or, in other words, they are refi-acted in passing from water into air.

^ What appears to me to have led Dr. R. astray in most of his reasonings is the singular opinion he seems to entertain, (although he has not expressly mentioned it,) that the rays of light do not proceed in all directions from every point of an object, but that they all go on in one particular direction pa- rallel to each other. For example, in his first experiment, he says, " Let us examine this experiment according to the

received

Observations on Rff) action. 7

received laws laid down in every elementary treatise on optics ; and I contend that no refraction or bending of the rays can possibly take place at d (see his figure), for tlie rays cd enter the air perpendicular to the plane surface of the water : con- sequently they must pass on without any refraction." Here the Doctor evidently supposes that only a cylinder of rays proceeds from the halt-crown perpendicular to the surface of the water. A number of his other arguments involve also this gratuitous assumption, " When the eye," says he, " is placed immediately over the half-crown looking down into the water, we see the image, not the piece of money, one-fourth nearer to the eye: here there can be no refraction, as the rays coming to the eye must be at right angles to the surface of the water : here there is no angle of incidence; no angle of refraction; no ratio of 3 to 4." Here the Doctor's reasoning is no doubt conclusive, if we admit him his owii principle which he has liere again assumed as an axiom ; for in this case the half- crown could not evidently be all seen at once, imless the pupil of the eye were at least as large as tlie half-crown itself; and taking the other parts of the eye proportional to this size of the pupil, we may safely conclude that no one since the crea- tion has been gifted with such organs of vision. In tl.e ex- periment which he gives with the prism, his alignments also hinge on the same principle. " Having." says he, " placed a sovereign under tlie plane ol' an equilateral jirism, I found ihat two reflected and not reiiacted images were fonned in each plane, as representetl in the following figure.

a the sovereign placed under the })laue dc of an equilateral }irism, forms an image at a; which image sends images to b i\\u\ /." Ac- cording to the present theory, two images '^ could not possibly be formed by refraction at h andy"; for a being at right angles to the plane d c, the rays should surter no refraction, but jirocced on to the vertex." Here the Doctor would have much obliged his mathematical readers, had he informed them what lie nieant by a ))oint being at right angles to a plane; but from his usual mode oi" rea- soning, we may suppose that he means the cijJinder of rays from the sovereinn rises at right angles to dc. However, that they are not reflected iniages may be made evident by turning the prism round on one ol its angles d or c, ior then the so- vereign will ajipear quite distorted and tinged with the pris- matic colours; whereas it is well known that any object seen by reflection from a plane surface never ajipears to bi' altered in shapo, but alwavs ])resorves its nafin-al lorm.

Dr. licade bccni*) surprised abo that Sir I.saac NcwU-n was

not

6 An Account of the Repeating Ci/cle,

not acquainted with the formation of two spectra when the r?vys of light were made to fall perpendicularly on one of its plane sides ; but I should suppose that he considered this as a self-evident corollary of the general principle of refraction. Indeed, if Newton had mentioned this as a particular disco- very, he might as well have told us that when two opaque bodies were intei-posed between the sun and a wall, there were also formed two shadows. Dr. R. says that " mathema- ticians are here obliged to relinquish one of their favourite laws, that rays striking at right angles to plane surfaces suffer no refraction ;" but he will here be pleased to recollect, that when rays fall at right angles to one of the faces, they must strike either of the other faces obliquely, and consequently be re- fracted at their emergence.

I should consider it an idle task to proceed any further in the refutation of doctrmes which do not carry along with them any thing like demonstrative evidence, but hinge entirely on the author's own ij)sc dixit ; my principal object being only to show the inconsistencies wliich result from the rejection of the law of refraction.

The Doctor has requested me to read his paper on Vision, published in a former Number of your Magazme ; but I sup- pose he must mean that which he published some tune ago in " The Annals of Philosophy," which I have also read ; but consider it quite foreign to the matter in question, whether the ideas of visible objects be conveyed to the mind by retinal or corneal images. I am, sir,

Your most obedient servant, H*K Majesty's Ship Queen Charlotte, CharlES StarK.

Portsmouth Harbour, May 26, 1822.

II. An Account of the Repeatiiig Ci7xle, and of the Altitude find Azimuth Instrument; describing their different Construc- tions, the Manner of performing their principal Adjustments, and ho-iV to make Observations with them .• together with a Comparison of their respective Advantages. By Edward Troughton, Esq. F.R.S., and Member of the American Philosophical Society*.

KJv all astronomical instruments, those fixed in national obser- vatories must be considered of the first importance to science; and in a commercial country, like our own, perhaps those sub- servient to nautical astronomy ought to be regarded as the next point in of utility. Those which I would call the third class are

* From the Memoirs of the Astronomical Society of London.

jiumcrous ;

and of the Altitude and Azimuth Instrument. 9

numerous ; they are such as are used in the small observatories of the amateur," to which they are in general equally adapted, as to the service of the gendeman who may travel to foreign parts. Of those, the two I have named in the tide, are the most approved of for these purposes; and to draw up a com- parison of their respective constructions and merits, is what I have chosen for the subject of this communication. Were I able to treat it as it deserves, I should entertain no doubt of its coming wthin the views of this Society, nor of its useful- ness ; particularly in assisting those, who may not already have become acquainted with the different kinds of instruments, in the selection of such as may be best suited to their purposes.

The repeating circle, till wiUiin these few years, has been very little used in this country, and in truth its merit but ill- appreciated ; facts however are not wantmg, although dispersed and insulated, sufiicient to remove all prejudice; particularly experiments recendy made, with a small instrument of this kind, at the principal stations of our grand national survey. On the continent of Europe, where the art of graduation is not so successfully cultivated as it is with us, an instrument which of all others depends the least upon accuracy of division, could hardly escape being too much commended: be this as it may, observations lately made on the other side of the British chan- nel, simultaneously with those used in the survey mentioned above, have I believe given the best inforaied of all parties a more correct idea of what may be expected from this instru- ment. T 1 1 T T

The altitude and azimuth instrument has 1 thmk been al- most exclusively made in this country: many of them have been sent abroad, but from their not havmg been used in great national operations, the advantage of them has seldom been made known to the world. Nearly the same may be said of those which remain at home ; for aUhough some of them have been much and skilfully used, yet owing to their having been only in the hands of private mdividuals, who had no common medium of communication, the labours of those who possessed them have hitlierto been almost lost to astronomy. From this general remark I must however except tlie observations of the ae brightest fixed stars, which Mr. Pond made at Westbury with a'^ao-inch circle of this kind, and which appeared m the Phil. Trans, for 1806. This indeed was tlie first thmg (not- witlistandiug some doubts and surmises from abroad) that un- equivocally demonstrated a change of figure in the Greenwich (juadrant, and subsequendy led to the procuration ot new in- struments for our national establishment.

The repeating circle has by no means failed for want of Vol.60. No. 291. Jr^/y 1822. B publicity;

10 An Account of the Repeating Circle,

publicity ; on the continent, astronomers and others have writ- ten a great deal about it, and the resuhs of thousands of ob- servations have been pubhshed ; the greater part of which were made on Polaris,- a star, to which, on account of its slow motion, this instrument is peculiar^ adapted. Although the altitude and azimuth uistrument, as a portable one, was pro- duced about the year 1792, we find no description of it in print, until the article Circle appeared in Rees's and Brew- ster's Encyclopedias*; the latter of which is referred to for the use of those who may wish to see a more detailed account of both the instruments under consideration, than can be given in the following brief descriptions.

Description of the Repeating Circle.

The lowest part of this instrument is a strong tripod, having at its extremities three steady foot-screws ; one of which, at least, should stand upon a well knov,-n apparatus, for the pur- pose of supplying a slower and finer motion to the upper part, than can be given by the screw itself. This apparatus should support that particular foot which during observation is di- rected to the meridian, or is opposite to the object observed. In the centre of the tripod is fixed a strong vertical axis, of a height sufficient for allowing head-room for observing con- veniently when the telescope is pointed towards the zenith. A pillar of the same height with the axis, is nicely fitted at both ends upon the latter, and both together, when the axis is ver- tical, produce a steady azimuthal motion. To the lower end of the pillar is fixed an azimuth circle: and to the higher end, a cross piece ; on the two extremities of which stand, about five inches apart, two upright bars for supporting a cross axis, to which the principal circle by its centre-work is attached, and round which axis the circle may be turned into any position from one side of the pillar to the other. A semicircle is fast- ened to one end of the cross axis, which, together with a clamp attached to one of the upright bars, affords the means of se- curing the circle in any position. The principal circle, or that of repetition, has (affixed to the middle of its plane, and opposite to the one divided) centre-work, the length of which

* It is true that the late Rev. Francis Wollaston, in the Phil.Trans. for 1 7f).3, gave a description of a two-feet circle which had an azimuth. That instru- ment, however, was solely designed for n meridian one, and was in fact quite unfit for any other purpose. The same gentleman, in the Appendix to his Fascicuhis, points out the best means of using an altitude and azimuth circle (pro])erly so called), but without giving any descrijjtion. The Wcstbury circle described in the Phil. Trans, for 1806, "although well constructed for obser\-ing azimuths, was not designed for taking transits, and besides was not a portable instrument.

I and of the Altitude and Azimuth Instrumeni. 11

is equal to about two-thirds of the diameter of the circle ; the outer part of which, being pertbrated from end to end, be- comes the socket for an axis of tlie same length, to which the index of the circle and telescope is attached in front. The index has tour branches placed at half right angles to the tele- scope, each of which subdivides the divisions of the circle into spaces of lO". To the middle of the cross axis is fixed a socket, which receives about two-thirds of the length of the centre- work: and the exterior surface of the remaniing third of that work becomes the axis for another telescope and a level to re- volve contiguous to the back of the circle. This is a com- plicated matter, difficult to be described or understood with- out a figure : it will however be sufficient, if it is conceived that there are tliree concentric motions in planes parallel to that of the circle: nameh', a general one within the socket of the cross axis, which carries round together, the circle, level and two telescopes : another, by wliich, upon the exterior part of the centre-work the level and back telescope revolve; and a third, that gives motion to the fore telescope and the verniers, so as to make them advance upon the circle, which is produced from the interior axis. These motions are independent of each other, and are all furnished wi|;h clamping and tangent screws. A counterpoise is placed upon the exterior end of the centre- work, which, by balancing the circle, telescopes, and level, keeps them stationary in any position. The greatest part of these instruments, M-hich have been constructed in London, have the back telescope on one side of the axis, and the level parallel to it on the other side, which latter, being made heavier than would be otherwise required, becomes a counterpoise for the former, a thing not attended to in the earlier constructions of the repeating circle. The azimuth circle of this instrument, only just named above, was in the first construction small, and of no other use than to point out roughly when the upper cir- cle had been tui'ned half round; but, in most of those made in London, to that circle has been given the snine radius, and the same attention paid to its execution as to the upper one. In the best construction of this part the circle is attached to the tripod, and tlu'ee indices fastened to the vertical pillar revolve round it; ihiis may a horizontal fingle be taken on three ecjuidistant parts of the circle, and, what is of ecjuai im- portance, by simpiv reversing the jiosition of the telescope and turning half roiuid in azimuth, a similar obstrvation maybe made, in which the readings will tall at 60" tlistauce from the former ones. Bv tliis double operation simple errf)rs of divi- sion may be considered as verv much diminished, ench sight

Vt 2 havmg

J^ An Account of the Repeating Circle^ -m

having been read off on six 'places ; and in both parts of the operation the error arising from eccentricity is, as to any sen- sible quantity, totally done away.

Description of the Altitude and Azimuth Instrument. The lower part of this instrument, like the other, consists of a tripod and feet-screws; which latter, being a recent contri- vance, and hitherto undescribed, may in this place deserve par- ticular notice. Each of the three screws is double ; that is, a screw within a screw : the exterior one, as usual, has its female in the end of the tripod, and the female of the interior screw is