88 COSMOS. 



i 



the terrestrial force, followed somewhat tardily the know- 

 ledge of the relations of the direction of this force in 

 horizontal and vertical planes (declination and inclination). 

 Oscillations, from the duration of which the intensity is 

 deduced, were first made an object of experiment towards the 

 close of the 18th century, and yielded matter for an earnest 

 and continuous investigation during the first half of the 19th 

 century. Graham, in 1723, measured the oscillations of his 

 dipping-needle with the view of ascertaining whether they 

 were constant, 92 and in order to find the ratio which the 

 force directing them bore to gravity. The first attempt to 

 determine the intensity of magnetism at widely different 

 points of the earth's surface, by counting the number of 

 oscillations in equal times, was made by Mallet in 1769. He 

 found, with a very imperfect apparatus, that the number of 

 the oscillations at St. Petersburg (59 56' N". lat.), and at 

 Ponoi (67 4'), were precisely equal 93 , and hence arose the 

 erroneous opinion which was even transmitted to Cavendish, 

 that the intensity of the terrestrial force was the same und-er 

 all latitudes. Borda, as he has himself often told me, was 

 prevented, on theoretical grounds, from falling into this error, 

 and the same had previously been the case with Le Monnier; 

 but the imperfection of the dipping-needle, the friction which 

 existed between it and the pivot, prevented Borda (in his 

 expedition to the Canary Islands in 1776), from discovering 

 any difference in the magnetic force between Paris, Toulon, 

 Santa Cruz de Tenerifte, and Goree in Senegambia, over a 

 space of 35 of latitude. (Voyage de La Perouse, t. i, 

 p. 162.) This difference was, for the first time, detected with 

 improved instruments in the disastrous expedition of La 

 Perouse in the years 1785 and 1787 by Lamanon, who com- 

 municated it from Macao to the Secretary of the French 

 Academy. This communication, as I have already stated, 

 (see p. 61), remained unheeded, and like many others lay 

 buried in the archives of the Academy. 



The first published observations of intensity, which more- 



02 Phil. Transact, vol. xxxiii, for 17241725, p. 332 ("to try if the 

 dip and vibrations were constant and regular"). 



93 Novi Comment. Acad. Scient. Petropol, t. xiv, pro anno 1769, pars 2, 

 p. 33. See also Le Monnier Lois du Magnetism* comparees aux observa* 

 tiontf 1776, p. 50. 



