B J R M E r E R. 211 



In order to prepare this barometer for an obfcrvation, the 

 one index is to be brought down, and the other raifed, until 

 both touch the knee of the float-wire. 



The next time the inflrument is obferved, the upper index 

 will point out the g^reateft depreffion of the mercury, or light- 

 nefs of the atmofphere, and the lower index the greateft rife of 

 the mercury or weight of the atmofphere, from the time the 

 fcale was prepared. 



By this means the variations of the atmofphere are much 

 more truly pointed out than by the common barometer. 

 For it often happens, that, during tempeftuous weather, or be- 

 fore it, the mercury both rii€s and falls within the fpace of a 

 few hours, or during the night time, which variations cannot 

 be difcovered by any of the barometers prefently in ufe. 



During the late very high winds, in November and Decem- 

 ber laft 1795, I have frequently obferved the mercury to rife 

 and fall within the fpace of two or three hours before the wdnd 

 begins ; and, during tempeftuous weather, it will fall very confi- 

 derably, and foon after rife higher than before, and ofcillate, or 

 rather undulate, upwards and downwards, the undulations be- 

 coming gradually lefs, until the atmofphere is more fettled ; 

 which fhows, that, like other fluids, when put in agitation, it 

 undulates till it come near an equilibrium ; for it appears fel- 

 dom to be in a ftate of perfe6l tranquillity .- 



Th e fudden fall and rife, or even the rife and fall of the mer- 

 cury, always denote an extraordinary agitation in the atmo- 

 fphere. And therefore, to foretell tempeftuous weather, it be- 

 comes of importance to obferve how many degrees the one in- 

 dex is removed from the other ; for example, at night, I take 

 note of the common barometer as ftanding at 29! inches, and 

 when I examine it in the morning find it at the fame height ; 

 from which I naturally conclude, that, as there has been no 

 agitation of the mercury, there will be calm or fettled wea- 



C c 2 ther. 



