164 BOOK OF NATURE LAID OPEN. 



and equiponderate that which is without, notwith- 

 standing the height of its column. 



The elasticity and resistance of the air, perhaps, 

 cannot be demonstrated by a more simple experi- 

 ment than by pressing with the hand on a bladder 

 with which it is confined; and the weight and pres- 

 sure of the atmosphere may be explained vv ithout the 

 aid of the air-pump, or other philosophical instrument, 

 merely by taking a common saucer, filled with wa- 

 ter, and turning down a teacup into it, with a piece 

 of flaming paper inside. As the fire destroys the in- 

 ternal air in the cup, a sort of vacuum will take place, 

 and the pressure of the outward air on the water in 

 the saucer will make it disappear and fill up. 



Although, in casting our eyes upwards, we do not 

 observe any boundary to the vast expanse, we are 

 not to consider that the atmosphere is unlimited. On 

 the contrary, we are taught, by the most rational cal- 

 culation, that if it extends much beyond forty -five or 

 fifty miles, it becomes so exceedingly -rare, as to be 

 unfit for the purposes for which the lower regions 

 are so well adapted. The atmosphere, at the height 

 of fifty miles, is said to be so rare, that it has no sen- 

 sible effect on the rays of light At the height of 

 forty-five miles it loses the power of refraction ; at 

 forty -one miles it is supposed to be rarified to that 

 degree as to occupy three thousand times the space 

 it does here; and Dr. Gregory observes, that it is 

 generally agreed that there are no clouds at the height 

 of four or five miles. 



