isr 



With us the mercury is highest when the wind is north 

 or north-east, and so brings the cold condensed air of 

 the northern climates. In all northern countries the 

 mercury varies more than in the southern, the winds 

 being more frequent, strong, various, and opposite to 

 each other. Between the tropics it scarce varies at all, the 

 "winds being ; small*, and generally blowing the same way. 



The pressure of the air is, coeteris paribus, as its 

 height. Carry the barometer to a higher place, where 

 the incumbent column of air is shorter, and a shorter 

 column of air is sustained : it being found to descend at 

 the rate of a quarter of an inch, for every hundred 

 feet of ascent. 



Now air, as all other fluids, must press equally every 

 way* Hence it is, that soft bodies sustain their pressure, 

 without any change of figure, and brittle bodies without 

 breaking, though that pressure be equal to that of a 

 column of mercury, thirty inches high, or a column of 

 water of thirty feet. Nothing can keep these bodies un- 

 changed but the equable pressure on all sides, which re- 

 sists as much as it is resisted; And hence on removing 

 or lessening that pressure on one side, the effect of it is- 

 soon perceived on the other. 



It is by means of its gravity, r. That the air closely 

 invests the earth with ail the bodies on it, and bends 

 them down : that it prevents the arterial vessels of 

 plants and animals from being too much distended by 

 the impetus of the circulating juices : and that it hinders 

 the blood from oozing out through the pores of their 

 containing vessels. Hence they who travel up high 

 mountains, the higher they ascend, are relaxed the more, 

 till, they fall into spitting of blood. 2. The mixture of 

 contiguous fluids is chiefly owing to this. Hence many 

 fluids which readily mix in the air, when that is removed, 

 remain separate. Q. It determines the action of one 

 body upon another. Thus it presses the particles of 

 fire against the fuel ; whereas upon removing the air, 

 the fire immediately goes out. So aqua regia ceases to 

 dissolve gold, if the air be taken away ; hence also on 

 the tops of high mountains, as on the Pike of Teneriffe, 

 the most acrid bodies, such as pepper, ginger, salt have 



