Pneumatics. 65 



and lighter than these vapours, and consequently 

 they can only ascend to a limited height. What 

 Ulloa observed on the Andes, has been confirmed 

 by the adventurers in balloons, and particularly 

 by Mr. Baldwin, who ascended from Chester in 

 the year 1785. The earth was entirely hid from 

 his view by the immense mass of vapours : he 

 compares them to a sea of cotton, tufting here 

 and there by the action of the air, and soon after 

 the whole became an extended floor of white 

 cloud. 



To prove the weight and pressure of the atmo- 

 sphere I shall mention an easy experiment, which 

 the student may make himself, without any phi- 

 losophical apparatus. If we nearly fill a com- 

 mon saucer with water, and then take a tea-cup, 

 and burn in it a piece of paper ; while the paper 

 is yet burning, turn down the cup, paper and all 

 into the saucer, we shall soon see that the pres- 

 sure of the air upon the water contained in the 

 saucer will force it up into the cup. To under- 

 stand the nature of this experiment it is necessary 

 to anticipate in some degree what will be the 

 subject of future lectures. Heat, caloric, or 

 fire, is now known to be a real substance; 

 when, therefore, the paper is burned in the tea- 

 cup, the air is driven out by another fluid (viz. 

 caloric) taking its place. Caloric, however, pe- 

 netrates all substances ; and therefore when the 

 flame is extinguished, it is dissipated through the 

 pores of the cup, leaving almost a perfect vacuum^ 



