PNEUMATICS. 187 



Do the laws of ^99. Air obeys the laws of motion which 

 t^°ik? "^^'^ ^rs commoa to all other material and ponder- 

 able substances. 

 Howisthemo- 400. The momentura of air, or the amount 

 Si^ijated^? ^"^ of force which it is capable of exerting upon 

 bodies opposed to it, is estimated in the same 

 way as in the case of solids, viz., by multiplying its weight 

 by its velocity. 



__T, ^ ... The momentum of air is usefully employed as a mechanioai 



WTiat are illus- -^ ^ •' 



tratious of the agent in imparting motion to wind-mills and to ships. Its 

 niomentum of ^^^^^ strikmg eflects are seen in the force of wuid, which oc- 

 casionaUy, in hurricanes and tornadoes, acts with fearfiil 

 power, prostrating trees and buildings. Such results are caused by the mo- 

 mentum of the air being greater than the force by which a building, or a tree 

 is fastened to the earth. 



401. Any force acting suddenly upon the air from a center, 

 the rings of imparts to it a rotary movement. A very beautiful illustra- 

 snioke observ- ^^^^ ^f- ^j^jg j^ ggg^ ^ ^j^g rinrra of smoke which are produced 

 ed in smoking ° "^ 



and in the dis- by the mouth of a skilful tobacco-smoker, and frequently also 



non"?^^ °^ '^°" upon a much larger scale by the discharge of cannon, on a 



still day. In these cases a portion 



"Ftp 1 S^ 

 of air acted upon suddenly from a center is caused 



to rotate, and the particles of smoke render the mo- vTv^T^'^'K^l*^^^ 



tion visible. The whole circumference of each \f^i^ ^'^-^ 



circle is in a state of rapid rotation, as is shown by vl./TT'/l' y^f^^h ' 



the arrows in Fig. 183. The rapid rotation in 



short, confines the smoke within the narrow limits of a circle, and causes the 



rings to be well defined. 



PRACTICAL PROBLEMS IN PNEUMATICS. 



1. If 100 cubic Inches of air weigh 31 grains, what will be the weight of one cubic foot T 



2. If the pressure of the atmosphere be 15 pounds upon a square inch, what pressure 

 will the body of an animal sustain, whose superficial surfiice is forty square feet ? 



3. When the elevation of the mercury in the barometer is 2S inches, what will be tho 

 height of a column of water supported by the pressure of the atmosphere ? 



Solution: Column of mercury supported by the atmosphere = 29 inches. Mercury 

 being l.lj^ times heavier than water, the column of water supported by the atmosphere = 

 13ix23=31 feet. 



4. When the elevation of the mercury in the barometer is 30 inches, what will be the 

 b«ight of a column of water supported by the atmosphere ? 



5. To what height may water be raised by a common pump, at a place where the ba- 

 rometer stands at 24 inches ? 



6 If a cubic inch of air weighs .30 of a grain, what weight of air will a vessel whose 

 capacity is (0 cubic inches, contain f 



