129 



have just described with the microeiectrometers 

 of Weiss, Gersdorf, and Marechaux*. These in- 

 struments discover electricity near a wall, in the 

 shade of a tree, every where almost, when Ben- 

 net's and Saussure's electrometers indicate none. 

 They are preferable to electric points fastened to 

 flying kites, or small balloons, because the elec- 

 tricity marked by these last, is most frequently the 

 mere result of the ascending motion, as has been 

 proved by the fine experiments of Mr. Erman-j-. 



I have had no better success than the majo- 

 rity of travellers in ascertaining the degree of 

 saltness of the sea^, which varies with the lati- 

 tude. From the small number of accurate ob- 

 servations I obtained by means of an areometer 

 by Dollond, differing little from that of Nichol- 

 son, it follows, that the specific gravity of the 

 sea water augments pretty regularly from the 

 coasts of Gallicia to TenerifFe, while it diminishes 



* Gilbert, Annalen, B. xv, p. 98. 

 + Ibid. p. 389 and 503. 

 % Mr. Proust, struck with the traces of mercury which he 

 had met with in all the muriats of soda of Spain (Nicholson's 

 Journ. of Nat PHI., 4to ed., vol. iii, p. 376), requested me, 

 at my departure from Madrid, to suspend, during the pas- 

 sage, a thin plate of gold or silver, to the poop of the vessel, 

 to see if it would offer any traces of amalgama. I followed 

 the advice of this celebrated chemist, though I had little con- 

 fidence in the success of this experiment j but the thread, 

 to which the plate was tied, broke a few days after I had put 

 my apparatus into the water. 



