VARIATION. 



complete year's obfcrvation ; from which it appears that the 

 diurnal variation increafes from January to June, and 

 decreafes from June to December. Phil. Tranf. vol. li. 

 p. 398, &c. 



It has alfo been obferved, that different needles, efpecially 

 if touched with different load-ftones, will differ a few mi- 

 nutes in tkeir valuation. See Poleni Epifl. Phil. Tranf. 

 N°42i. 



We fhall here fubjoin a method praftifed by M. Du Hamel, 

 who was one of thofe who attempted, and fucceedcd in the 

 preparation of artificial magnets, for enlarging the fcale of 

 the variation. At each extremity of the needle, compofed 

 of two magnetic bars, and which is fourteen inches long, a 

 ilender pointed piece of lte;;l is crefted perpendicularly ; and 

 at the diilance of fifty-two feet, in the diredlion of the 

 needle, he has placed on two pillars, and in a line perpendi- 

 cular to that direction, a graduated limb fix feet long ; being 

 a fegment of a fnppofed circle, defcribed from the centre on 

 which the needle turns. The obferver, placing himfelf fo 

 as to bring the two pieces of fteel at the extremities of the 

 bar into a hne with the eye, obferves where that line pro- 

 longed, or the vifual ray, points to the graduated arc. As, 

 at this diftance, each of thefe degrees meafures a foot, the 

 true direction of the needle is afcertained with the greateft 

 precifion ; and left the obferver's eyes may not be good 

 enough to enable liim to fee diflinftly the particular divifions 

 at that diftance, an aflillant occafionally moves a certain in- 

 dex, conformably to his direftion. Hift. de I'Acad. Roy. 

 des Scienc. Paris, for 1772, part ii. Mem. 2. 



On the EffeB of the heal /lttra3'ton of the Ship upon the 

 ' F'ariation of the Needle. — We have already had occafion to 

 notice the neceffity of attending to the diredion of the fhip's 

 head, in pbfervations made on fhip-board relative to the 

 dircAion of the compafs ; and that this may have a very 

 fenfible ctfeft will appear very obvious, when we recoUedl 

 the quantity of iron wuth which a (hip of war, in particular, 

 is generally loaded ; and that this is moftly forward, while the 

 compafs is generally aft. The great attraftion between tlie 

 iron and the needle is generally known ; and confequently, 

 if we could imagine the magnetic power of the earth to 

 ceafe entirely, we lliould have no difficulty in conceiving 

 that the attraftion of the guns, &c. would incline the needle 

 to affume a direftion coinciding with that of the veffel ; and 

 confequently, when the magnetic meridian and the direftion 

 of the (hip are the fame, that is, when the veffel lies nortli 

 and fouth, both forces adting in the fame manner, the pofi- 

 tion of the needle will be the fame as if no fuch local at- 

 traftion exifted. But if the (hip's head is put over to the 

 eaft or the weft, then the local attraftion of the fhip will in- 

 cline the needle to the eaft and weft, while the terrcftrial 

 attraftion will draw it towards the north ; and it will, there- 

 fore, affume a direftion which correfponds with tlie re- 

 fultant of thefe two diftinft forces : and we may obferve, 

 that the direftion of this refultant would furnifh, if well ob- 

 ferved, moft important data towards determining the inten- 

 fity of this myfterious power. 



Simple and obvious as this idea is, it does not appear to 

 have been formed till Mr. Wales, the aftronomer in captain 

 Cook's voyages, was ftruck with certain irregularities, 

 which he could in no way at firft account for, and of which 

 we have the following account in the Introduftion to the 

 attronomical obfervations in the Second Voyage. 



" In the Enghfh Channel, the extremes of the obferved 

 variation were from 19^° to 25°; and all the way to the 

 Cape of Good Hope, I had frequently obferved differences 

 nearly as great, without being able in any way to account 

 for them ; the difference in the fituation being by no means 



fufBcient. Thefe irregularities continued after leaving the 

 Cape, which at length put me upon examining into the cir- 

 cumftances under which they wei-e made. In this examina- 

 tion it foon appeared, that when moft of thefe obfervations 

 were made, wherein the greateft variation had happened, 

 the (hip's head was north and eajlcrly ; and that when thofe 

 where it was the lead had been taken, it was fouth and 

 lueflerly. I mentioned this to captain Cook, and fome of 

 the officers, who did not at firll feem to think much of it ; 

 but as opportunities happened, fome obfervations were made 

 under thofe circumftances, and very much contributed to 

 confirm my fufpicions ; and throughout the whole voyage, 

 I had good reafon to believe that variations obferved, with 

 the Jlo'p's head in different pofttions, and even in different parts 

 of her, will differ very materially from one another, and 

 much more will obfervations made on board different (hips, 

 which I now find fully verified, on comparing thofe made 

 on board the Adventure with my own, made about the fame 

 time in the Refolution." 



Mr. Wales again recurs to this fubjeft, in the courfe of 

 his aftronoraical obfervations made in Cook's third voyage, 

 and points out the quantity of the deviation in feveral cafes ; 

 yet the true caufe of thefe anomalies does not appear to have 

 luggefted itfelf to this able aftronomer : he merely ftates the 

 refults, but offers no explanation of them. Nor does it ap- 

 pear that they were afterwards particularly noticed by any na- 

 vigator, till captain Fhnders'a attention was called to the fub- 

 jeft in the early part of his laft voyage. Here that experienced 

 navigator found fuch unaccountable differences in the quan- 

 tity of variation, deduced from the different obfervations 

 he occafionally made, that he determined on inftituting an 

 inquiry into their caufes ; and, if poflible, to afcertain the 

 laws by which they were regulated. 



" Several inftances," he obferves, " have been mentioned 

 in the courfe of this voyage, where the compafs fhewed a 

 different variation, on being removed from one part of the 

 fhip to another. Thus, obfervations on the binnacle gave 

 29-5° off the Start, where the true variation was 25^° ; 

 whilft others taken from the booms before the main-maft, 

 68 miles lower down the Channel, gave only 24° ; and in 

 the experiments made with five compaffes, the mean varia- 

 tion on the binnacle was 4° 37' greater than on the 

 booms. 



" It foon became evident, however, that keeping the 

 compafs to one fpot was not alone fufficient to fecure accu- 

 racy : a change in the direftion of the fhip's head was alfo 

 found to make a difference in the needle ; and it was necef- 

 fary to afcertain the nature and proportional quantity of this 

 dift'erence, before a remedy could be applied. This inquiry 

 was attended with many difficulties, and no fatisfaftory con- 

 clufion could be drawn, until a greater variety of obferva- 

 tions was coUefted. It then appeared, that when th^ 

 fhip's head was on the eaft fide of the meridian, the dif- 

 ferences were moftly one way ; and when on the weft fide, 

 they were the contrary : whence I judged that the iron in the 

 fhip had an attraftion on the needle, which drew it forward. 

 But there was this remarkable diftinftion : in the northern 

 hemifpliere, it was the north end of the needle which was 

 attrafted ; and in the fouthern hemifphere, it was the fouth 

 end. In the inilances off the Start, before cited, when the 

 fhip's head was -weff, the north end of the needle had been 

 drawn forward, or to the left of the north, nearly 4°, and 

 the weft variation thereby increafed to 29^° ; with the head 

 at eaff, it would be drawn to the right of its natural pofition, 

 and the variation diminifhed to about 2l|°; but at north, 

 the attraftion of the (hip was in the fame line with the mag- 

 netic poles of the earth, and would, therefore, produce no 

 4 1 z change, 



