-\ GUIDE FOR KEEPERS OF POULTRY 197 
For some reason not easy to understand the cheaper 
styles of the incubators now on the market are built on 
the hot-water plan and the high-priced ones are hot-air 
machines. Curiously enough our great egg and duck 
farms almost invariably use hot-air machines, while the 
farmers and smaller operators seem inclined to buy hot- 
water machines. While it is true that a good many thou- 
sand hot-air machines are sold singly to farmers and own- 
ers of small poultry plants, we do not know of a single 
large poultry plant equipped with hot-water incubators. 
Many devices for regulating the temperature have been 
tried, but all have been discarded by the modern incu- 
bator manufacturer, except the style that controls the 
heat-valve by a device which takes advantage of the ex- 
pansion of metals and the one which is controlled by the 
expansion of ether confined within a brass disc made with 
two sides, so that when the heat causes the ether to ex- 
pand the sides of the disc are forced apart, suitable mech- 
anism operating a system of levers to cut off the heat 
when this occurs. Here again is a sharp distinction. 
Most if not all of the hot-water incubators use the disc 
regulator, while all the hot-air machines, which have be- 
come standard, except two or three, use the expansion 
of metals for regulating. 
The modern incubator, whether hot-air or hot-water is 
used in heating, has arrived at a stage where its economic 
value is acknowledged; it has become one of the essen- 
tials of the well-equipped poultry plant, and is rapidly 
coming to be considered indispensable on the farm, as it 
saves time and labor and expense. The incubator makes 
it possible to hatch a large number of chicks at the same 
time, giving them an equal chance to grow up, bringing 
them to market size at the same time and allowing of 
