126 
American Milk- Condensing Factories. 
at the time of expulsion, to its density when introduced into the 
cylinder ; and for the same reason it is of equal importance that 
the stroke of the pump be made as long as convenience will allow, 
in order to lessen the effect of the contents of this dead space. 
The piston moves up to the valve-plate, to within one-sixteenth 
of an inch, which space is always filled by a small quantity of 
water, admitted at the water-cock T. The valves lie down flat 
with the plate, so that very little loss is occasioned by them, 
while the stroke of the pump is made long for the reason above 
specified. 
The pump-valves are of peculiar construction, having been 
devised by Mr. A. S. Cameron, of New York City. It is well 
known that valves made from metal alone cannot be kept tight 
for any length of time, as the pounding of metal on metal soon 
renders their surfaces uneven ; while, on the other hand, valves 
made wholly of rubber are destroyed by being forced into the 
openings of the valve-seats, making it necessary to renew them 
frequently. In this valve both of these difficulties are overcome, 
by combining both metal and rubber in its construction. The 
body of the valve consists of a casting of brass, or any other 
suitable metal, provided with an annular groove or dove-tailed 
recess, corresponding with the shape of the valve-seat that the 
valve is intended to cover. This recess in the valve is filled 
with indiarubber in a raw (?r plastic condition, which is then 
vulcanised while in its place at a temperature of 350° Fahr., 
allowing it to project a sixteenth of an inch beyond the metallic 
surface. 
From this it will be seen that, when in operation, the pro- 
jecting rubber comes in contact with the smooth surface of the 
valve-seat, making a perfect joint, while the body of the rubber, 
being confined in the groove, is incompressible, and, in fact, 
indestructible by ordinary agents. And then a valve is ob- 
tained combining in itself all the good qualities of metal and 
rubber to give strength and tightness, while the objectionable 
features of a valve made wholly from either are overcopie. 
The lining of the vacuum-cylinder, its piston, piston-rod, valves, 
and valve-seats, are all made of composition, to prevent any 
injurious effects from the acids with which they may come in 
contact. The whole machine, in fact, has been carefully studied, 
and is well adaj)ted to the duty for which it is intended. 
Having now described the latest improvements in machinery 
for evaporating in vacuo, it only remains to give an illustration 
(Fig. 6), showing Messrs. Guild and Garrison's pan and pumps, 
now in use at some of the factories, and with it 1 shall close this 
branch of my subject. 
