Abbildungen der Seite
PDF
EPUB

1819.

[blocks in formation]

Jan. 3.372
Feb. 2.191
Mar. .758
April 4.175
May 1.238
June
July

.975 6.8 6.5

6.7 33.8 32.7 33.3.146.139.142 86.4 87.1 86.7
.750 7.5 7.2 7.4 31.7 30.4 31.1.136.130.133 84.5 84.1 84.3
1.800 15.9 11.5 13.7 34.3 33.4 33.8.149.144.146 73.7 78.7 76.2
1.840 19.2 10.8 15.0 36.0 35.7 35.9.156 .155.156 72.0 81.2 76.6
2.650 25.7 11.3 18.5 39.3 40.7 40.0.180.182,181 67.6 82.9 75.3
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
1.256 2.735 28.0 13.4 20.7) 51.9 50.2 51.1.264 .248.256 74.0 84.6 79.3
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
Sept. 1.523 1.950 21.5 13.1 17.3 46.9 46.6 46.8.224.224.224 75.9 82.9 79.4
Oct.
4.015 1.430 14.2 11.3 12.7 39.5 38.4 38.9.187.180.183 77.7 80.2 78.9
Nov. 1.518 .825 9.3 6.7 8.0 30.9 30.8 30.9.132.132.132 81.3 85.5 83.4
Dec. 1.654 .665 5.4 5.2 5.3 29.5 28.2 28.9.127.121.124 87.2 88.3 87.8

Avr. 25.625 20.785 17.2 10.1 13.6 39.7 38.9 39.3.186.179.182 77.5 83.9 80.7)

[blocks in formation]
[blocks in formation]

HYGROMETER.

Leslie.

Point of

Anderson.
Moisture in

Deposit.

100 C. I.

Relative Humidity.

Jan.

52.0 21.0 17.0 4.0 30.360 28.690 1.100 .050 17 0 44.6 22.4.204 .097 100.0 72.6 29.900 28.770 .748 .030 22 0 42.4 21.0.191.082 100.0 52.0 1.133 .030|| 29 1 44.6 12.0 .204 .068 98.0 41.0 .453.005 38 0 45.6 22.4 .211 097 100.0 48.0 .365.005 47 3 53.0 7.0.268 .057 96.0 27.0 .630.020 555 54.8 31.4 .284 .132 94.0 42.0 .355 .025 50 5 64.0 38.4.381.167 95.0 54.0 .375 .015 51 3 64.6 43.0.386.194 97.0 53.4 .540.010 40 2 59.4 32.4.328.137 98.0 52.4 .460.030 27 1 57.6 15.4.309 .076 99.0 47.0 .685.110 26 0 43.4 19.0.197.087 100.0 51.0 .705.010 17 0 47.4 18.6.225.086 100.0 62.0

Feb. 49.5 21.0 16.5 5.0
Mar. 55.0 26.0 18.0 5.0
April 58.0 31.0 21.0 5.5
May 63.0 30.0 250 5.5
June 68.0 41.5 23.0 18.0
July 74.0 42.5 22.5 8.5
Aug. 79.0 47.0 19.0 8.0
Sept. 67.5 38.0 23.0 4.5
Oct. 63.0 26.5 22.5 3.0
Nov. 50.0 20.5 19.5 5.5
Dec. 51,5 9.5 19.0 2.0

30.144 28.892
30.150 28.935
30.060 29.415
30.190 29.175
30.235 29.170
30.335 28.740
30.420 29.020
30.230 29.225
30.165 28.980
30.295 28.840

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 1818 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

its full effect on the water. The comparison, therefore, between 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,

BARLOW ON MAGNETIC ATTRACTIONS." *

THE variation of the compass, a subject at all times no less interesting to the philosopher, than useful to the navigator, was first discovered, we are told, by Columbus, in his voyage to America in 1492; and Professor Gillebrand of Gresham College, in 1625, ascertained that this variation was itself of a changeable nature. The discovery of these important and very extraordinary facts, excited a lively interest, among men of learning and science, throughout all Europe; the exertions of our celebrated countryman, Dr Halley, on this subject, are well known, and need not be here repeated.

The phenomenon of the magnetic dip, or inclination of the needle, accidentally discovered by Norman in 1592, was also then a subject of much speculation and inquiry; and, to render this law in the magnetic system subservient to science and navigation, the latitude, in any given meridian, was attempted to be ascertained by its results; but, the delicacy of the instrument, and experience, very soon proved its demonstrations erroneous; and, until the last voyage of captain Flinders, was adverted to more as a matter of curiosity to philosophers, than of utility to navigators.

The diurnal variation of the compass, first discovered by Mr Graham, who has been followed by Mr Weirgentin, Mr Canton, and, last of all, the indefatigable exertions of Colonel Beaufoy, likewise excited considerable attention; but though numerous theories have been formed to account for this phenomenon, none, as yet, have appeared satisfactory to philosophers, or useful to science, if we except Mr Barlow's theory, which we here intend shortly to notice.

From the beginning of the 18th, to the 19th century, this very important, and highly useful, branch of science,

R. G.

was allowed to slumber; since nothing of the least consequence, during that long and enlightened period, was added to our previous knowledge, on this interesting subject. It is, indeed, true, that several distinguished navigators observed, during their respective voyages, anomalies in the variation of the compass, altogether inexplicable; and, what appears very extraordinary, the more pains that was taken by them to discover them, the further did they go away in point of theory. Dampier, when off the Cape of Good Hope, where the variation was truly estimated at 11°, was much puzzled, and, no doubt, greatly perplexed, to find only 7° 38'. Mr Wales, in his second voyage with captain Cook, was surprised and astonished to find, in the English Channel, and indeed throughout the voyage, a difference in the quantity of variation, though observed with the greatest care, of 3°, 4o, 5o, 6o, 7°, and even 10°. Captain Phipps, afterwards Lord Mulgrave, during his voyage towards the north pole, found the like differences, which he attributed to the inaccuracies of the compasses. "We made," says he, 66 se❤ veral observations, which we found, by those taken at seven in the afternoon, to be 17° 9' west; by others, at three in the afternoon, only 7° 47' west: I could not account for this very sudden and extraordinary decrease," &c. Monsieur Beautemp Beaupré, while in search of the unfortu nate La Perouse; Captain Vancouver; and many others, found the like errors of variation, without being able, in any way whatever, to account for them; until Captain Flinders, that acute and penetrating, but unfortunate, man, in his last voyage of discovery to Terra Australis, in 1801, 1802, and 1803, first discovered the true cause producing 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,

1820.

cordances in the variation on ship board-a change in the direction of the ship's head. Having ascertained this most important truth, it soon occurred to him, that a local attraction must exist in the ship; which, in connexion with terrestrial magnetic attraction, acted on the magnetic needle, when placed at the binnacle, with a compound force; and, therefore, he found by experiments, that in the northern hemisphere, when the head was at west, this combined attractive power drew the north end of the needle to the west; and in the southern hemisphere, to the east, of the true magnetic meridian; This difference, produced by local attraction, he denominated the Deviation.'

Finding the maximum deviation in both hemispheres, when the ship's head was at west or east; and that the needle stood right when the head was in a direction with the magnetic meridian-north and south ;-what was the proportion of deviation, he asked himself, at the intermediate points, between the east and west and magnetic meridian? After much labour and consideration, it appeared to him, "that the errors produced by local attraction should be proportionate to the sines of the angles between the ship's head and the magnetic meridian; and, therefore, in order to find this proportion, it seemed probable the following Rule would be found applicable in all parts of the world, viz. "that the error produced at any direction of the ship's head, would be to the error at east or west, at the same dip, as the sine of the angle between the ship's head and magnetic meridian was to the sine of eight points, or radius.

Captain Flinders dying soon after his return to England, we see no further attempts made either to verify or overthrow the accuracy of this rule, until 1807, when a small practical work, entitled, an Essay on the Variation of the Compass, by W. Bain, master in the royal navy," was pub

66

lished. In this valuable tract* the author, in the last section, has, in a variety of examples wholly incompatible with the supposition of truth, completely exposed the fallacy of this rule-which Captain Sabine fully corroborates by observations made during the late arctic expedition; and which, indeed, is the only thing of the least consequence done in that voyage that has added to our previous knowledge on this subject. As to the unintelligible paper written by Mr Scorsby, and inserted in the Philosophical Transactions for 1820, Part I., we are compelled to say, it is wholly undeserving of the least attention, and serves no other end whatever than to bewilder the reader into a labyrinth of useless experiments, which, we much question, whether the author himself rightly understands. How very different are the experiments and deductions we now intend briefly to analyse !

Mr Barlow, author of the work before us, an able mathematician, and one of the Professors of the Royal Military Academy, Woolwich, sensible of how very much real importance a formula, founded on correct principles, for correcting the deviation produced by a change in the direction of the ship's head, in all approachable latitudes, would be to science and navigation, and, indeed, to mankind in general, has at length arrived at the conclusion, after a long, laborious, and patient investigation of the laws of magnetic attraction, which his situation and place afforded the most ample opportunity and means for experiments, no less honourable to himself than beneficial to science and practical navigation. Before we introduce our readers into our author's workshop, it may be proper they should clearly understand what Mr Barlow's ideas were, on this subject, before he commenced operations; and the theory on which all his future hopes depended. We shall transcribe his own words :-" Since the iron of the vessel," 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 good deal, and cannot well be accounted for.

pass must be supposed to maintain the same relative position, with respect to each other, during the voyage, I imagine it to be possible to place a single ball of iron, equal to the whole mass, in a certain situation of the ship, when its effects upon the needle would be the same as that of the iron in its distributed state; or what amounts to the same thing, that all the forces acting on the needle in the actual state of the iron, may be reduced to a single resultant. I then assume, that a less mass of iron (having its entire attraction, or resultant, in the same line as the former,) may be approximated so near to the compass as to produce an effect equal to that of the iron of the vessel, whereby the tangent of the angle of deviation may be at any time doubled, and hence the deviation itself determined. Under this point of view, however, a slight computation would be requisite; but since the tangent of small ares have very near the same ratio of the ares themselves, we may suppose the angle itself doubled by the experiment, and hence the deviation ascertained by observation only." Preface, p. 5 and 6. Though our author's first experiments rather involved him into difficulties, at least proved nothing at the time, yet it is necessary the reader should know the apparatus he worked with, as well as the method by which he worked with it; I began," he says, p. 3, "by describing, on a platform, several concentric circles, from eight to sixteen inches radius, draw ing through the centre a line, in the direction of the magnetic meridian; I then set off my east and west points; and, lastly, divided the whole circle into equal parts of 10° each." With the compass over the centre of these concentric circles, he passed round successively, on each circle, several shells of different diameter; which, as our author anticipated, produced results at the time wholly inexplicable. At length, however, our author, having exchanged his platform for a strong table, divided according to the points of the compass, with a circular hole of ten inches diameter in the centre, through which a ten inch shell was, at pleasure, made to pass by means of a block and pully, he again commenced operations, by passing the compass on the circle round the ball, instead, as before, the ball round the

This

compass; "I elevated the ball," he continues, "till its action was imperceptible'; and then gradually lowering it, I noticed the deviation at various altitudes of the ball, with the compass of each point of division on the circles; observing also very accurately the height or depth of the centre of the ball above or below the pivot of the needle, when the deviation was zro. These results, indeed, were the only ones applicable to my present inquiry; and from them I ascertained that the several parts of no action were all situated in one plane; the inclination of the plane itself to the horizon being found nearly equal to 20°, declining directly from the magnetic north point to the south. plan is, therefore, either exactly or very nearly perpendicular to the direction of the dipping needle. The formula used by our author for computing the result, which will be found in page 19, and which approximates nearly to that found by experiment, is this:— tan. I.-sec, a. When I denotes the inclination, r the radius of the circle, which, in the present instance, was twenty inches, h the observed height or depth of the centre of the ball, and a the angle from the east or west points of the circle. The mean result of all those calculations, which involves too many figures to be inserted in this outline, gives 19° 24'.

h

reosa

h

The different manifestations indicated in these important experiments, between the needle and the attractive power of the ball, at different angles, horizontally and perpendicularly, induced our author to believe, that there were in every ball of iron two planes, in which the compass may be anywhere posited, without being influenced in its direction; the one that of no attraction, and the other the vertical plane, corresponding to the magnetic meridian. In consequence of which, he conceived an ideal sphere to be circumscribed about the ball of iron; and assuming the circle of no attraction as an equator, and the poles of that circle as the poles of the sphere, he imagined circles of latitude and longitude to be described round the ball in several circles, keeping it always at the same distance from the centre; and, therefore, when these ideal circles of latitude and longitude were cor

rectly ascertained by calculation, and verified by experiment, as particularly described page 24, and to which we must refer our readers, he succeeded in establishing the law of deviation as it respects the latitude; namely, that the tangents of the deviations are proportional to the rectangle of the sine and cosine of the latitude; or to the sine of the double latitude, which is the same thing. In the same way our author establishes a like law of elevation, as it respects the longitude; and, to explain himself more fully, he has given two diagrams, one describing the laws of deviation as it regards the latitude, and the other as it regards the longitude.

With regard to the laws of attraction as it regards the mass of iron, our author has, by a great variety of experiments, verified by computation, clearly demonstrated, that the power of attraction resides wholly on the surface, and is independent of the mass; or, in other words, that the tangents of the deviations are proportional to the cubes of the diameter, or as the power of the surface, whatever may be the weight or thickness.-Page 48.

The striking confirmation of the existence of the plane of no attraction in the most irregular masses of iron; and that the power of attraction resides wholly on the surface, and is independent of the mass; as exhibited by experiment, and confirmed by computation, must have gratified and encouraged our author in his toilsome and unbeaten path; and, therefore, to put the matter beyond all dispute, he determined to verify his former experiments by others of a different nature, and on a much larger scale. Accordingly he addressed a letter to Sir William Congreve, requesting permission to pursue his inquiries in the repository at Woolwich; and, it gives us great pleasure to say, that the very polite and handsome manner in which this communication was answered, reflects the highest honour on the character of that worthy and ingenious gentleman. How contrary was the behaviour and conduct of the Royal Society to our author on a similar occasion, will best be seen in Mr Barlow's own language, page 12; which will appear totally incomprehensible when contrasted with the answer of Sir William, and the manner in which a memorial, addressed by our author, for the like purpose,

to the Board of Admiralty, was acknowledged; and the pleasing and friendly way Sir George Cockburn, one of the Admiralty Lords, and Mr Croker, the secretary, offered our author all their interest in the further prosecution of his experiments.

It is many years since we knew Captain, now Admiral Cockburn, and our readers will easily conceive, that the man who, with the same activity and thirst after professional knowledge, could ascend the masthead, and there detect stupidity in the adjustment of a sky-sail studdingsail, as descend the hold, and superintend the stowage of a water-cask, must have felt exquisite pleasure and delight on entering our author's workshop, and there behold, by accurate experiment, the true cause producing the extraordinary anomalies in the variation of the compass. Mr Croker, too, a man of science, liberality, and erudition, could not help feeling, on this occasion, much gratification; and it is the highest encomium we can pay to the professional knowledge of the one, and learning of the other, to have it in our power thus to record an instance of such noble generosity and kindness in behalf of aspiring talent and genius.

But, to return from this digression, our author procured, for his next experiment, an iron 24 pounder, mounted on a platform which admitted of its being traversed through an entire circumference; the tracks at the bottom running over a circle ten feet six inches diameter, divided into 32 equal parts corresponding with the points of the compass. A piece of wood, projecting four feet from the muzzle of the gun, for the compass to stand on, was made to fit exactly the bore of the gun, on which the compass could be moved to any distance, at the time of experiment. As it is impossible to make room for the results, we shall just now say, that they fully corroborated the accuracy of the results given by former experiments; and that, in the present instance, the difference between the observed and the computed results is so very trifling, as to be almost imperceptible. It is necessary, however, the reader should know the formula by which our author obtained results bearing so close affinity with those given by experiment. "The computed deviations," he observes, "was

« ZurückWeiter »