3o5 Poji/o-ipt to Valla's Letters 



Thefe Inftrumeuts are now made at Berlin, Weimar, 

 Drefden, Vienna, and Paris, On coming from a cold to a 

 very vvarra temperature the fcrew cd muft be loofed before 

 you approach the fire. This inftruraent has an advantage 

 by which it is diftinguirtied from all others : it is the only 

 one in which the fize of the Torricellian vacuum depends on 

 the obferver : the height of the mercury remains the fame 

 to whatever depth it be plunged in the mercury, provided 

 you take cire each time to adjull the level by the cock. 

 Mufcati has proved, that the barometric height is affefted 

 by the fize of the vacuum on account of the attraction of the 

 glafs In k, and becaufe the 7)ihiimu7n of air which the va- 

 cuum contains is more or lefs dilated- In Humbolt's ba- 

 rometer, the fame Torricellian vacuum exifts on the tops of 

 the mountains as in the valleys. 



The diflcrent pieces of which this inftrument confifts are 

 contained in one cylinder, which may be carried on the 

 ihoulder like a fuzee. Several tubes may be contained alfo 

 in a walking-ftick. Having for two years meafured the 

 height of different mountains with Humbolt's barometer, 

 and others^ conftrueled on the old principles, no fenfible 

 ditt'erences were obferved, though this operation, for various 

 rcafons explained by Tremblay, is ftill very far from perfec- 

 tion. 



XVI. Pcjlfcript to Folt^s Letters on Animal Ekdincilji *. 



S. 



'OME new fafts, lately dilcovered, fecm to fliew that the 

 immediate caufe which excites the eleftric fluid, and puts it 

 ill motion, whether it be an attraftive or a repulfive power, is. 

 to be afcribcd much rather to the mutual contact of two dif- 

 ferent metals, than to their contacl with moitl condu£lors. 

 But, though it cannot bQ denied that in the latter cafe there 

 exifts. an adion, it is proved that it exerts itfelf in a far more 



• See page 59 and i6j. 



confider- 



