68 
pi=-6345 / 
We have the reduced velocity 
U„- =650 (feet per sec.) (16) 
Therefore the discharge will be given in cubic inches per 
sec., KO being the effective area of the orifice by 
piQ = 12U„p„KO I ^ 
= 12 x 650KO j 
Or since the actual area in square inches 
0 = 000314 sq. inches 
Q = 2*44K (cubic inches per sec.) (18) 
10. In order to compare the experimental discharges with 
those calculated it is necessary to know besides the size of 
an orifice and the pressure and temperature of the dis- 
charofing vessel — the coefficient of contraction or the effec- 
tive area of the orifice. To obtain this form the equations 
require that the terms depending on viscosity should be 
introduced which render the integration so far impossible. 
The only plan is to obtain this coefficient by comparing the 
theoretical results with the experimental. Such compari- 
sons have been made by Prof. Weisbach for air and in the 
case of short cylindrical orifices such as that used by Mr. 
Wilde (a cylindrical hole through a plate having a radius 
equal to the thickness of the plate), the value of K the 
coefficient of contraction given by Wiesbach (Applied Mecha- 
nics List, 254, Pankine) is from ’73 to ‘833. Whether these 
are the real coefficients of contraction may, however, well be 
doubted, as it is so extremely difficult to determine the 
experimental quantities of gas discharged owing to the great 
effect of slight variations of temperature on the relations 
between changes of pressure and changes of temperature, 
such changes of temperature being almost necessarily inci- 
dental on changes of pressure. 
11. In Mr. Wilde’s experiments the pressure was allowed to 
