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ACCOUNT OF 



might be supposed to arise from the action of the 

 Sun and Moon. A very sensible instrument was 

 obviously necessary for such a purpose ; and I was 

 therefore led to the idea of measuring the pressure 

 of the atmosphere by its effects in compressing a 

 column of common air, Upon constructing an in- 

 strument of this kind however, I found that the 

 air was absorbed by the fluid with which it was in- 

 closed, and that a good and permanent barometer 

 . could not be made upon such a principle till this 

 radical defect was removed. I therefore directed 

 my attention particularly to this object, and suc- 

 ceeded beyond my most sanguine expectation, in 

 freeing the Air Barometer from this great source of 

 inaccuracy. 



The name of Sympiesometer which 1 have given 

 to this improved instrument, is derived from the 

 Greek words .(rvfMrtsfyj to compress, and pergov a 

 measure, denoting the property it possesses of mea- 

 suring the weight of the atmosphere by the com- 

 pression of a gaseous column. 



The principle of the Sympiesometer, which is re- 

 presented in one of its forms, in Plate II. Fig. 2., 



* When I constructed this instrument, I was not aware 

 that Dr Hooke had employed the compression of a column 

 of air to measure the weight of the Atmosphere. The 

 Sympiesometer, however, will be found to have no resem- 

 blance to his instrument but in this particular. 



