PROPERTIES OF MATTER IN THE GASEOUS STATE. 
775 
the same as the rate at which the other firing goes on, but that the velocity of the 
shot from H is just as much greater than the mean velocity, as the velocity of the shot 
from C is below the mean. Then it is at once seen that the total tendency on H is 
to move back, while the total tendency on C is to move forward. 
It obviously follows from the foregoing that the inequality in the forces on H and C 
could only occur at a certain distance from their ends, which distance would depend 
on the distance between the batteries ; and hence that the ratio which this inequality 
(due to any particular rate of firing) would bear to the wdiole reaction on either 
battery would increase as the length of the batteries diminished; or in other words, 
the inequality of force would be proportional to the distance between the batteries, 
and would be constant whatever might be the length of the batteries beyond a 
certain point. 
At first sight it may appear that the distance between the batteries H and C should 
be analogous to the distance between the hot and cold plates; but it is necessary to 
remember that it is only in case of the gas being extremely rare, as compared with 
the distance between the plates, that the molecules can be supposed to go straight 
from the one plate to the other. In ordinary cases the molecules encounter other 
molecules, and the effect of such encounters is to reduce the motion to a mean. 
Hence it appears that the distance between the batteries as affecting the equality 
in the reactions is somewhat analogous to the distance which a molecule may be 
supposed to travel without losing its characteristic motion. And hence it would 
appear that in the case of gas the inequalities of force on the two plates would be 
proportional to the inverse density of the gas and the extent of the boundaries of 
plates. 
57. The shot from H which miss C, and those from C which miss H, must be 
stopped by the outside batteries. Therefore the inequalities in the forces on H and 
C will be balanced by inequalities in the forces on the batteries behind, and the sum 
of the forces on H and the battery behind will be equal to the sum of the forces on C 
and the battery behind. 
And this is strictly analogous to the result of Shuster’s experiment, viz. : that 
the effect upon the vanes of the light mill is exactly balanced by the effect on the 
containing vessel. 
58. The batteries also serve to illustrate the action of thermal transpiration. In 
the case already considered (Art. 57) the inequality between the shot from H which 
miss C and those from C which miss H is transferred to the outside batteries, or in the 
case of the gas, to the containing vessel. The better to illustrate the present point, 
suppose that the outside batteries are ranged across the ends of the open space 
between H and C. This will make no difference to the result. The inequality of the 
action of the shot which miss H and C must now cause a force parallel to the end 
batteries, tending to cause these batteries to move end-wise in the direction of C. 
Suppose that the tw r o batteries H and C were free to move together in the 
MDCCCLXXIX. 5 G 
