218 Mr. ©. HE. S. Phillips on a 
to it from M, and M, cut it in A, and Ay. Cut off from 
the perpendicular through A, a length A,N,=m, and from 
that through A, (produced if necessary) a length AgNo=m, 
in the opposite direction to A,N,. Join N,N, and let it cut 
A,A,in C. Then, as before, M,C and CM, are the momenta 
of the spheres at the end of the first portion of the impact. 
Measure from © along N,N, in the direction of Ba 
length CF=1], and in the opposite direction CH=e. Join 
BF and draw ED parallel to BF. M,D and DMs, are in 
direction and magnitude the momenta of the two spheres 
after impact. 
The velocities are found from them by dividing by the 
masses. 
XXIV. A new Automatic Gas-Pump. 
By C. E. 8. Paruurrs*. 
HIS apparatus is constructed upon a plan which enables 
the pump, when once set in operation, to continue 
automatically and to produce as perfect a Torricellian vacuum 
as is possible. 
It has been devised with a view to providing a compara- 
tively portable machine suitable for special laboratory work 
or for researches requiring prolonged pumping, and consists 
of three distinct parts, viz. :—a small motor-driven mechanical 
pump, a four-way control valve, and a modified Toepler 
apparatus by which the final vacuum is obtained. The 
action is as follows :—-A vessel (say a Rontgen-ray lamp) 
having been sealed on to the Toepler pump, the motor is 
set in motion, and a vacuum produced in the whole appa- 
ratus equivalent to within about °*5 inch of the barometric 
column. 
The mercury now fills the lower chamber L (fig. 1) of the 
Toepler and completes an electrical circuit, which includes 
an electromagnet capable of so moving the controlling valve 
that it allows air at atmospheric pressure to enter and drive 
the mercury slowly into the upper chamber U. The Toepler 
pump operates in the usual manner. At the instant the 
quantity of mercury, moving as described, diminishes in L, 
the circuit just completed is broken, but the mercury soon 
accumulating in the trap T “ makes ”’ a second cireuit which 
moves the slider of the valve back to its original position. 
* Communicated by the Physical Society : read April 22, 1904, 
