213 
external atmosphere, the conoidal form of the orifice in- 
creasing the amount of rarefaction above that obtained with 
a plain tube orifice. This conclusion is further evident on 
comparing the times of discharge from D in reversed 
positions from a pressure of 31bs. to lib. ; for as the rarefac- 
tion in the vacuum chamber was only reduced to 1-5 inches 
of mercury, the phenomenal change in the rate of discharge 
again presents itself, making a difference of 17 seconds in 
the times of discharge between the reversed position of the 
orifice for the lowest pressure. 
Comparing the times of discharge through the tube orifice 
A and the orifice 0 in the thin plate it will be seen that 
there is much less difierence between them than for the 
same orifices in Table IL, the ratio agreeing very closely 
with those shown in Table I. for similar times of discharge. 
The approaching equality in the times of discharge through 
the tube orifice A and the orifice in the thin plate for the 
lower pressures is no doubt due to the friction of the 
issuing stream of air against the sides of the tube orifice. 
The effect of this friction for the lowest pressure, as will be 
seen, reduces the rate of discharge from the orifice A below 
that from the orifice in the thin plate. 
From the results of my previous experiments on the dis- 
charge of atmospheres of higher into atmospheres of lower 
density, the times and coefficients in Table I. and Table III. 
for the higher pressures may well be considered as having 
been obtained for discharges into a perfect vacuum, the 
difference in the coefficients for pressures below lOlbs. in 
Table III. being entirely due to friction of the issuing stream 
of air against the sides of the orifices. 
From the results shown in Tables I. and II. the maximum 
rate of efflux is obtained from the orifices A, B, and C, and 
taking the efflux from these orifices as unity, the value of 
the coefiicient for the efflux of air into a vacuum through an 
orifice in a thin plate is *937. 
