HISTORY OF 



it. There are also frequent changes in the air, 

 without any sensible alteration in the barometer. 



As the barometer is thus used in predicting the 

 changes of the weather, so it is also serviceable in 

 measuring the heights of mountains, which mathe- 

 maticians cannot so readily do : for as, the higher 

 we ascend from the surface of the earth, the air 

 becomes lighter, so the quicksilver in the baro- 

 meter will descend in proportion. It is found to 

 sink at the rate of the tenth part of an inch for 

 every ninety feet we ascend ; so that in going up 

 a mountain, if I find the quicksilver fallen an 

 inch, I conclude, that I am got upon an ascent 

 of near nine hundred feet high. In this there 

 has been found some variation ; into a detail of 

 which it is not the business of a natural historian 

 to enter. 



In order to determine the elasticity of air, the 

 wind-gun has been invented, which is an instru- 

 ment variously made ; but in all upon the prin- 

 ciple of compressing a large quantity of air into 

 a tube, in which there is an ivory ball, and then 

 giving the compressed elastic air free power to 

 act, and drive the ball as directed. The ball, 

 thus driven, will pierce a thick board ; and will 

 be as fatal at small distances, as if driven with 

 gunpowder. I do not know whether ever the 

 force of this instrument has been assisted by 

 means of heat; certain I am, that this, which 

 could be very easily contrived by means of phos- 

 phorus, or any other hot substance applied to 

 the barrel, would give such a force as I doubt 

 whether gunpowder itself could produce. 



