124 American Milk-Condensivg Factories. 
injurious action. This is especially the case if the pan be 
worked continuously, bu.t, if long stoppages are made, copper 
possesses decided advantages over cast iron. Under ordinary 
circumstances, however, the cast-iron vacuum-pans answer every 
purpose, and they can be constructed in such a perfect manner 
that one similar to that described will retain a vacuum of 
29 inches for a space of twelve hours without losing more than 
one inch. 
The Improved Vacuum-Pump. — I have said that when milk, 
solutions of sugar, and most organic substances, are to be eva- 
porated, it is found necessary to expose them to as low a tem- 
perature as possible, so as to avoid decomposition, and for this 
purpose the boiling in vacuo, in a so-called vacuum-pan, is of 
especial advantage. The vacuum is produced by a pump, which 
first removes the air from the pan, and afterwards the vapour 
arising from the boiling liquid. Figs. 4 and 5, on the next page, 
represent a vacuum-pump, which, for correctness of the prin- 
ciples involved, and for the manner in which they have been 
carried out, is worthy of special attention. " It has," says the 
Technologist, from which I have taken my description, " already 
been subjected to the most severe tests in many sugar-houses." 
Hitherto a great deal of annoyance has been experienced, owing 
to the bad proportions and worse construction of many machines 
applied to this purpose, as a falling off in the vacuum affects the 
result very materially, and any stoppage of the machine owing to 
a break-down, &c., arrests the whole operation, and entails a 
serious loss. This machine is, strictly speaking, an independent 
vacuum-pump, and as it is confined to the duty of forming 
a vacuum, it may be worked at that speed which will afford 
the very best result without interfering in any way with any 
other operation. 
In the accompanying engravings. Fig. 4 is a side elevation, 
the pump-cylinder being partly in section, to show the valve- 
plate and valves, and Fig. 5 is a plan, similar letters indicating 
the same parts in both figures. A is the steam-cylinder ; B 
the valve-chest, arranged as is usual in a steam-engine ; C the 
steam pipe ; D the exhaust-pipe. The engine is a so-called 
back-acting one, tbe crank-shaft being placed between the 
cylinder and the cross-head. This involves a necessity for two 
piston-rods, E E \ the former passing over the shaft and the latter 
underneath it. F is the cross-head; F^ the slides; G the con- 
necting-rod ; H the crank and crank-shaft; the pillow 
blocks ; I the fly-wheel. J is the pump piston-rod, which is 
extended beyond the piston, and passes out on the other end of 
the pump-cylinder through a stuffing-box, the piston being 
consequently carried at two points, so as to reduce as much as 
