204 HISTOr.Y OK 



rain than it was before it. There are ako 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 is it also serviceable in nieasiu'ing the heights of 

 mountains, which mathematicians cannot so readily do : for, as 

 the higher we ascend from the surface of the earth the air be- 

 comes lighter, so the quicksilver in the barometer 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 conchide, 

 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 nivented, which is an instrument vai-iously 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 com 

 pressed elastic air free power to act, and drive the ball as di- 

 rected. 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 veiy easily contrived by means of phosphorus, or any 

 other hot substance applied to the barrel, would give such a force 

 as J 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 instru- 

 ment, 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 retiu-ning into the vessel again. But this, 

 like all other complicated instruments, will be better understood 

 by a minute inspection, than an hour's description : it may suf- 

 iice here to observe, that by depriving animals, and other sub- 

 stances, of all air, it shows us what the benefits and effects of 

 air are in sustaining life, or promoting vegetation. 



The digester is an instrument of still more extraordinary ef- 

 fects, than any of the former ; and sufficiently discovers the 

 amazing force of air, when its elasticity is augmented by fire. 

 A common tea-kettle if the spout were closed up, and the lid 



