Abbildungen der Seite
[blocks in formation]
[ocr errors]

Jan. 3.372 .975 6.8 6.5 6.7 33.8 32.7 86.4 87.1 86.7
Feb. 2.1911 .750 7.5 7.2 7.4 31.7 30.4 84.5/84.1 843
Mar. .758 1.800 15.9 11.5 13.7 34.3 33.4 73.7 78.7 76.2
April 4.175 1.840 19.2 10.8 15.0 36.0 35.7 72.0 81.2 76.6
May 1.238 2.650 25.7 11.3 18.5 39.3 40.7 67.6 82.9) 75.3
June 2.617 2.800 28.4 13. 1 20.8 45.8 44.3 45.0.216 .204.210) 69.9 82.6) 76.3
July 1.256 2.735 28.0 13.4 20.79 51.9 50.2 61.1 .264.248.256 74.0 84.6 79.8
Aug. 1.308 2.365 24.3 11.2 17.8/ 57.3 55.4 56.4 .310 .292.301 79.8 88.5 84.1
Oct. 4.015 1.430 14.2 11.3 12.7 39.5 77.7 80.2 78.9
Nov. 1.518 .825 9.3 6.7 8.0 30.9 30.8 81.3 85.5 83.4
Dec. 1.654/ .665 5.4 5.2 5.3 29.5 28.2 28.9.1271.121.124 87.288.3 87.8

[ocr errors]

4:673 1:250 11.5 12., 13:396.8 36.0 231 73.9 82.0 78.00

Avr. 25.625 20.785 17.2 10.1 13.6 39.7/38.9 77.5) 83.9 80.7

[blocks in formation]
[ocr errors]

Jan. 52.0 21.0 17.0 4.0 || 30.360 28.690 1.100.050 17 0 44.6 100.0 72.6 Feb. 49.5 21.0 16.5 5.0 | 29.900 28.770 748.030 220 42.4 100.0 52.0 Mar. 55.0 26.0 18.0 5.0 30.144 28.892 1.133.030 29 144.6 98.0 41.0 April 58.0 31.0 21.0 5.5 30.150 28.935 .453 .005 38 0 45.6 100.0 46.0 May 63.0 30.0 25.0 5.5 30.060 29.415 365.005 47 3 53.0 96.0 27.0 June 68.0 41.5 23.0 10.0 30.190 29.175 .630.020 55 554.8 31.4 .284.132 94.0 42.0 July 174.0 42.5 22.5 8.5 30.235 29.170 355 .025 50 5) 64.0 38.4 .381.167 95.0 54. Aug. 79.0 47.0 19.0 8.0 30.335 28.740 375.015 51 3 64.6 43.0-386 194 97.0 53.4 Sept. 67.5 38.0 23.0 4.5 30.420 29.020.540 .010 40 2 59.4 32.4 .328.13798.0 52.4 Oct. 63.0 26.5 22.5 3.0 30.230 29.225 460 .030 27 1 57.6 15.4.309.070 99.0 47.0 Nov. 50.0 20.5 19.5 5.5 30.165 28.980 .685.110 260 43.4 100.0 51.0 Dec. 151,5 9.5 19.0 2.0 || 30.29528.840 705.010) 17 | 47.4 100.0 63.4

It appears from the above tables, that the mean temperature of 1819 is about seven-tenths of a degree lower than that of 1818; the mean height of the Barometer :014 higher; the quantity of rain 1772 less; and the mean of Leslie's Hygrometer 2 higher. The mean daily range of the Thermometer and Barometer is almost exactly the same for both years. The quantity of evaporation exceeds that of 1818 by 729 of an inch. The mean point of deposition, at 10 a. m. is about half a degree lower than the mean minimum temperature, but the coincidence is sufficiently exact to demonstrate the accuracy of Mr Anderson's principles.

Another observation which has been often alluded to in the pages of your Magazine, and which I think an important one, is amply confirmed by the preceding abstract. I allude to the coincidence between the mean of the daily extreme temperatures, and the mean of the temperatures at 10 morning and evening. The difference for the whole of 1819, amounts only to two-tenths of a degree. For several years preceding I found the mean difference three-tenths.

The temperature of spring-water taken three times a-month, and which gave a result for 1816 coinciding almost exactly with the mean of the daily extremes, is 1.5 degree higher for 1819. It is to be remembered, however, that the great cold of December, which reduced the mean temperature of the air considerably below the usual average for the season, has not yet produced

[ocr errors]
[ocr errors]

its full effect on the water. The comparison, therefore, bétween the two, ought not to be made till that season of the year when the temperatures of the air and the exterior of the earth approach one another, which takes place about the month of May. I have no doubt that then the coincidence will be nearly exact. -I am, Sir, your obedient servant,

R. G.

[ocr errors]

BARLOW ON MAGNETIC ATTRACTIONS. * The variation of the compass, a sub- was allowed to slumber ; since nothing ject at all times no less interesting of the least consequence, during that to the philosopher, than useful to the long and enlightened period, was adnavigator, was first discovered, we are ded to our previous knowledge, on told, by Columbus, in his voyage to this interesting subject. It is, indeed, America in 1492; and Professor Gil- true, that several distinguished navis lebrand of Gresham College, in 1625, gátors observed, during their respece ascertained that this variation was it- tive voyages, anomalies in the variaself of a changeable nature. The dis- tion of the compass, altogether inexcovery of these important and very ex- plicable; and, what appears very extraordinary facts, excited a lively in- traordinary, the more pains that was terest, among men of learning and taken by them to discover them, the science, throughout all Europe ; the further did they go away in point of exertions of our celebrated country- theory. Dampier, when off the Cape man, Dr Halley, on this subject, are of Good Hope, where the variation was well known, and need not be here re- truly estimated at 11°, was much peated,

puzzled, and, no doubt, greatly perThe phenomenon of the magnetie plexed, to find only 70 38. Mr dip, or inclination of the needle, acci- Wales, in his second voyage with dentally discovered by Norman in 1592, captain Cook, was surprised and

asto was also then a subject of much spe- nished to find, in the English Chane pulation and inquiry; and, to render nel, and indeed throughout the voythis law in the magnetic system sub- age, a difference in the quantity of servient to science and navigation, the variation, though observed with the latitude, in any given meridian, was greatest care, of 3o, 4o, 5o, 6o, 7o, attempted to be ascertained by its re and even 10°. Captain Phipps, aftersults ; but, the delicacy of the instrum wards Lord Mulgrave, during his ment, and experience, very soon prove voyage towards the north pole, found ed its demonstrations erroneous; and, the like differences ; which he attria until the last voyage of captain Flin- buted to the inaccuracies of the comders, was adverted to more as a matter passes. “We made,” says he,“ of curiosity to philosophers, than of veral observations, which we found, utility to navigators.

by those taken at seven in the after: The diurnal variation of the com noon, to be 17° 0' west; by others, at pass, first discovered by Mr Graham, three in the afternoon, only 7° 47' who has been followed by Mr Weir- west: I could not account for this gentin, Mr Canton, and, last of all, very sudden and extraordinary dethe indefatigable exertions of Colonel crease," &c. Monsieur Beautemp BeauBeaufoy, likewise excited considerable pré, while in search of the unfortuattention ; but though numerous the- nate La Perouse ; Captain Vancouver, ories have been formed to account for and many others, found the like errors this phenomenon, none, as yet, have of variation, without being able, in any appeared satisfactory to philosophers, way whatever, to account for them; or useful to science, if we except Mr until Captain Flinders, that acute and Barlow's theory, which we here intend penetrating, but unfortunate, man, in shortly to notice.

his last voyage of discovery to Terra From the beginning of the 18th, to Australis, in 1801, 1802, and 1803, the 19th century, this very important, first discovered the true cause produ. and highly useful, branch of sciences cing these hitherto unaccountable dis

An Essay on Magnetic Attractions : Particularly as respects the Deviation of the Compass, occasioned by the Local Influence of the Guns, &c. With an Easy Practical Method of observing the same in all parts of the World. By Peter Barlow, of the Royal Military Academy. Printed for J. Taylor, Architectural Library, Holborn, London,


Variation of the Compass, by W. Bain, words :-" Since the iron of the ves

cordances in the variation on ship lished. In this valuable tract* the board change in the direction of author, in the last section, has, in a the ship's head. Having ascertained variety of examples wholly incompathis most important truth, it soon oc tible with the supposition of truck, curred to him, that a local attraction completely exposed the fallacy of this must exist in the ship; which, in con- rule—which Captain Sabine fully cornexion with terrestrial magnetic ata roborates by observations made during traction, acted on the magnetic needle, the late arctic expedition; and which, when placed at the binnacle, with a indeed, is the only thing of the least compound force; and, therefore, he consequence done in that voyage that found by experiments, that in the has added to our previous knowledge northern hemisphere, when the head on this subject. As to the unintelia was at west, this combined attractive gible paper written by Mr Scorsby, and power drew the north end of the inserted in the Philosophical Transaca needle to the west ; and in the south- tions for 1820, Part I., we are comern hemisphere, to the east, of the pelled to say, it is wholly undeserving true magnetic ineridian ; This differ- of the least attention, and serves no ence, produced by local attraction, he other end whatever than to bewilder denominated the Deviation."

the reader into a labyrinth of useless Finding the maximum deviation in experiments, which, we much ques. both hemispheres, when the ship's tion, whether the author himself head was at west or east; and that rightly understands. 4 How very difthe needle stood right when the head ferent are the experiments and deducwas in a direction with the magnetic tions we now intend briefly to analyse ! meridian-north and south ;-- what Mr Barlow, author of tħe work bewas the proportion of deviation, he fore us, an able mathematieian, and asked himself, at the intermediate one of the Professors of the Royal Mix points, between the east and west and litary Academy, Woolwich, sensible magnetic meridian? After much la- of how very much real importance & bour and consideration, it appeared to formula, founded on correct principles

, him, “ that the errors produced by for correcting the deviation produced local attraction should be proportion- by a change in the direction of the ate to the sines of the angles between ship’s head, in all approachable latithe ship's head and the magnetic me- tudes, would be to science and naviridian ; and, therefore, in order to gation, and, indeed, to mankind in find this proportion, it seemed pro- general, has at length arrived at the bable the following Rule would be conclusion, after a long, laborious

, found applicable in all parts of the and patient investigation of the laws world, viz. that the error produced of magnetic attraction, which his sia at any direction of the ship's head, tuation and place afforded the most would be to the error at east or west, ample opportunity and means for exat the same dip, as the sine of the angle periments, no less honourable to himbetween the ship’s head and magnetic self than beneficial to science and pracmeridian was to the sine of eight points, tical navigation. Before we introduce or rudius,

our readers into our author's workCaptain Flinders dying soon after shop, it may be proper they should his return to England, we see no fur- clearly understand what Mr Barlow's ther attempts made either to verify or ideas were, on this subject, before he overthrow the accuracy of this rule, commenced operations; and the theory until 1807, when a small practical on which all his future hopes dependwork, entitled, “ an Essay on the ed. We shall transcribe his own master in the royal navy, was pub- sel,” says our author, " and the com

The importance of this work, in a practical point of view, having excited considerable interest, we refer such of our readers as may not have had an opportunity of seeing the book itself, to Brand's Journal of Science and the Arts, No 7,-Monthly Review, November 1817,--and British Review, November 1819,_who have each treated the work and the author in a popular manner. Indeed, it has always been to us a matter of regret, that Mr Bain should not have been employed in the late Arctic expedition,

+ That this paper should have been inserted, and that of Mr Barlow's rejected, in that No of the Philosophical Transanctions, surprises us a geod deal,

and cannot well be acom counted for

[ocr errors][ocr errors][ocr errors]
[ocr errors]





reosat sec, a.

[ocr errors]
[ocr errors]

pass must be supposed to maintain the compass ; " I elevated the ball," he same relative position, with respect to continues, " till its action was impereach other, during the voyage, I ima- ceptible; and then gradually lowergine it to be possible to place a single ing it, I noticed the deviation at various ball of iron, equal to the whole mass, altitudes of the ball, with the compass in a certain situation of the ship, when of each point of division on the cire its effects upon the needle would be cles ; observing also very accurately the same as that of the iron in its dis- the height or depth of the centre of tributed state; or what amounts to the ball above or below the pivot of the same thing, that all the forces acte the needle, when the deviation was ing on the needle in the actual state These results, indeed, were the of the iron, may be reduced to a sin- only ones applicable to my present gle resultant. I then assume, that a inquiry; and from them I ascertained less mass of iron (having its entire ate that the several parts of no action traction, or resultant, in the same were all situated in one plane; the line as the former,) may be approxi« inclination of the plane itself to the mated so near to the compass as to horizon being found nearly equal to produce an effect equal to that of the 20o, declining directly from the magiron of the vessel, whereby the tan- netic north point to the south. This gent of the angle of deviation may be plan is, therefore, either exactly or very any

time doubled, and hence the nearly perpendicular to the direction of deviation itself determined. Under the dipping needle.

The formula used this point of view, however, a slight by our author for computing the reune computation would be requisite ; but sult, which will be found in page

since the tangent of small ares have 19, and which approximates nearly to

very near the same ratio of the ares that found by experiment, is this :ned themselves, we may suppose the angle

When I

tan. I.= itself doubled by the experiment, and hence the deviation ascertained by ob- denotes the inclination, r the radius servation only.” Preface, p. 5 and 6. of the circle, which, in the present Though

our author's first experi- instance, was twenty inches, h the ments rather involved him into diffi- observed height or depth of the cenculties, at least proved nothing at the tre of the ball, and a the angle from time, yet it is necessary the reader the east or west points of the circle. should know the apparatus he worked The mean result of all those calculawith, as well as the method by which tions, which involves too many figures he worked with it;-" I began,” he to be inserted in this outline, gives says, p. 3, "by describing, on a plat- 19° 24'.

form, several concentric circles, from The different manifestations indie is eight to sixteen inches radius, draw- cated in these important experiments, hinten ing through the centre a line, in the between the needle and the attractive

direction of the magnetic meridian ; I power of the ball, at different angles, then set off my east and west points; horizontally and perpendicularly, ina and, lastly, divided the whole circle duced our author to believe, that there into equal parts of 100 each.” With were in every ball of iron two planes, the

compass over the centre of these in which the compass may be anyconcentric circles, he passed round where posited, without being influil successively, on each circle, several enced in its direction; the one that of

shells of different diameter; which, no attraction, and the other the veras our author anticipated, produced tical plané, corresponding to the magresults at the time wholly inexplicable. netic meridian. In consequence of

At length, however, our author, have which, he conceived an ideal sphere ingexchanged his platform for a strong to be circumscribed about the ball of table, divided according to the points iron ; and assuming the circle of no of the compass, with a circular hole attraction as an equator, and the poles of ten inches diameter in the centre, of that circle as the poles of the sphere, through which a ten inch shell was, he imagined circles of latitude and lona at pleasure, made to pass by means of gitude to be described round the ball a block and pully, he again commen in several circles, keeping it always at ced operations, by passing the com the same distance from the centre; pass on the circle round the ball, in- and, therefore, when these ideal circles stead, as before, the ball round the of latitude and longitude were cor


[ocr errors]

rectly ascertained by calculation, and to the Board of Admiralty, was alverified by experiment, as particularly knowledged ; and the pleasing and described page 24, and to which we friendly way Sir George Cockburn; must refer our readers, he succeeded one of the Admiralty Lords, and Mr in establishing the law of deviation as Croker, the secretary, offered our auit respects the latitude ; namely, that thor all their interest in the further the tangents of the deviations are pro prosecution of his experiments. portional to the rectangle of the sine It is many years since we knew and.cosine of the latitude ; or to the Captain, now Admiral Cockburn, sine of the double latitude, which is the and our readers will easily conceive, same thing. In the same way our au- that the man who, with the same thor establishes a like law of elevation, activity and thirst after professional as it respects the longitude; and, to knowledge, could ascend the mastexplain himself more fully, he has head, and there detect stupidity in given two diagrams, one describing the the adjustment of a sky-sail studdinge laws of deviation as it regards the la sail, as descend the hold, and supertitude, and the other as it regards the intend the stowage of a water-caske

, longitude.

must have felt exquisite pleasure and With regard to the laws of attrac- delight on entering our author's work, tion as it regards the mass of iron, our shop, and there behold, by accurate

, author has, by a great variety of ex- experiment, the true cause producing periments, verified by computation, the extraordinary anomalies in the clearly demonstrated, that the power variation of the compass. Mr Croker

, of attraction resides wholly on the sure too, a man of science, liberality, and face, and is independent of the mass ; erudition, could not help feeling, on or, in other words, that the tangents this occasion, much gratification; and of the deviations are proportional to the it is the highest encomium we can pay cubes of the diameter, or as the power to the professional knowledge of the of the surface, whatever may be the one, and learning of the other, to have weight or thickness.--Page 48. it in our power thus to record an in

The striking confirmation of the ex stance of such noble generosity and istence of the plane of no attraction in kindness in behalf of aspiring talent the most irregular masses of iron; and and genius. that the power of attraction resides But, to return from this digression, wholly on the surface, and is inde our`author procured, for his next expe; pendent of the mass; as exhibited by riment, an iron 24 pounder, mounted experiment, and confirmed by coma on a platform which admitted of its putation, must have gratified and ene being traversed through an entire circouraged our author in his toilsome cumference; the tracks at the bottom and unbeaten path ; and, therefore, to running over a circle ten feet six inches put the matter beyond all dispute, he diameter, divided into 32 equal parts determined to verify his former expe- corresponding with the points of the riments by others of a different na- compass. A piece of wood, projecting ture, and on a much larger scale. Ac- four feet from the muzzle of the gun, cordingly he addressed å letter to Sir for the compass to stand on, was made William Congreve, requesting permis- to fit exactly the bore of the gun, on sion to pursue his inquiries in the re- which the compass could be moved to pository at Woolwich; and, it gives us any distance, at the time of expert great pleasure to say, that the very polite ment. As it is impossible to make and handsome manner in which this room for the results, we shall just now communication was answered, reflects say, that they fully corroborated the aca the highest honour on the character of curacy of the results given by former that worthy and ingenious gentleman. experiments; and that, in the present How contrary was the behaviour and instance, the difference between the conduct of the Royal Society to our observed and the computed results is author on a similar occasion, will best

so very trifling, as to be almost imbe seen in Mr Barlow's own language, perceptible. It is necessary, however, page 12; which will appear totally in the reader should know the formula comprehensible when contrasted with by which our author obtained results the answer of Sir William, and the bearing so close affinity with those manner in which a memorial, addresse, given by experiment.

« The comed by our author, for the like purpose, puted deviations," he observes,

[ocr errors][merged small][ocr errors][ocr errors][ocr errors][ocr errors]


« ZurückWeiter »