124 A HISTORY OF 



ry-nine inches, when the air is heavy ; but not above twenty-six, when 

 the air is very light. Thus, by this instrument, we can, with some 

 exactness, determine the weight of the air ; and, of consequence, tell 

 before-hand the changes of the weather. Before fine dry weather, 

 the air is charged with a variety of vapours, which float in it unseen, 

 and render it extremely heavy, so that it presses up the quicksilver ; 

 or, in other words, the barometer rises. In moist, rainy weather, the 

 vapours are washed down, or there is not heat sufficient for them to 

 rise, so that the air is then sensibly lighter, and presses up the quick- 

 silver with less force ; or, in other words, the barometer is seen to 

 fall. Our constitutions seem also to correspond with the changes of 

 the weather-glass ; they are braced, strong, and vigorous, with a large 

 body of air upon them ; they are languid, relaxed, and feeble, when 

 the air is light, and refuses to give our fibres their proper tone. 



But although the barometer thus measures the weight of the air 

 with exactness enough for the general purposes of life, yet it is often 

 affected with a thousand irregularities, that no exactness in the instru- 

 ment can remedy, nor no theory account for. When high winds blow, 

 the quicksilver generally is low : it rises higher in cold weather than 

 in warm ; and is usually higher at morning and evening than at mid- 

 day : it generally descends lower after rain than it was before 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 moun- 

 tains, which mathematicians 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 baromter 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 instrument variously made; but in all upon the 

 principle 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 distance's, 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 phospho- 

 rus, or any other hot substance applied to the barrel, would give such 

 a force as I doubt whether gunpowder itself could produce. 



The air-pump is an instrument contrived to exhaust the air from 

 round a vessel adapted to that purpose, called a receiver. This 

 method of exhausting, is contrived in the simple instrument, by a 

 piston, like that of a syringe, going down into the vessel, and thus 

 pushing out its air; which, by means of a valve, is prevented from re- 

 liirning into the vessel again. But this, like all other complicated 



