244 VELOCITY OF EFFLUX OF AIR. 



temperature is lowered, and when the air is compressed the 

 temperature is increased. Under these circumstances the 

 pressure cannot change according to Art. 48, and other 

 formulae have to be produced. (See Weisbach's Mechs., p. 

 936; also Ency. Brit, Vol. XII., p. 480.) 



129. Velocity of Efflux of Air According to 

 Mariotte's Law. Let the air be discharged from an 

 orifice with the velocity v feet per second; let iv = the 

 weight of a cubic foot of air, and v l = the volume of air 

 discharged per second; then the work performed by the 

 volume of air v^ in passing from the pressure p^ to the 

 pressure p z , is, by (2) of Art. 128, 



and this must be equal to the work stored in the air during 

 the efflux, which is 



Therefore, we have 



v* wv* p* 



-*% - = p.V l log , 



l} gl. (1) 



* Pz 



A cubic foot of air, at the temperature of the centi- 

 grade thermometer, and at a pressure corresponding to the 

 height of 29.92 inches of the barom'eter, weighs about 

 0.08076 Ibs.* Therefore for any temperature t, we have 

 for the weight of a cubic foot of air, from (2) of Art. 54, 

 since for the same volume and temperature the weight 

 varies as the density, 



0.08076 



* Determined by Regnault. See WeiBbach'a Mechs., p. 795. 



