56 
The Illustrated Book of Poultry. 
expanded in the tube, which was balanced on an axis or pivot, it caused the axis to turn, and 
a lever attached to it actuated the valves. More recently, Mr. Halsted has introduced various 
important modifications into his incubator, which is now known in America as the “ Centennial.” 
The arrangement for bringing hot water to the outside of the tank is retained ; but we have 
already mentioned that the wet flannel is abandoned. The regulator was also found defective, 
the heavy bulb of mercury often causing fracture, and the mercury after a time becoming 
oxidised. A compound thermostatic bar, which bends with the heat of the drawer, is therefore 
substituted, and gives good results. 
Many other incubators have been constructed in America, most of them self-regulating. 
Probably the most perfect yet made is that of Mr. E. S. Ren wick, a mechanical engineer and 
patent agent, which embodies several original patents. He also employs a thermostatic bar, made 
either of strips of brass and vulcanite fastened together, or of two different metals. When these 
move sufficiently, they set in motion, by releasing a detent, a clock or engine actuated by weights, 
and with a “ balance ” constructed like a small paddle-wheel, working in glycerine, which opens 
valves and actuates the lamp at the same time. The eggs are laid between rollers which form the 
bottom of the drawer, and which, when revolved, “ turn ” all the eggs at once. The lamp is not 
at first actuated by the regulator, but when more heat is required a flue of otherwise waste heat is 
directed through the tank, so as to increase the temperature, a method which we shall see has 
been employed in the most recent machines of English make. The most striking novelty about 
this incubator, however, is the whole system of working it. The temperature is not meant or 
attempted to be kept uniform ; but the clock is so arranged that after the incubator has been 
kept a few hours at the lowest temperature of some given range — about three degrees — the higher 
temperature is kept up for about the same time. In this way the inventor considers the processes 
of Nature to be more closely imitated, and states that much greater extremes of heat can be 
borne under such circumstances, no° having been withstood with impunity; the regulator and 
clock reducing the temperature after a few hours. For this view we shall find hereafter some 
reason ; and there can be no doubt that this thermostatic incubator, like Mr. Halsted’s last make, 
has hatched remarkably well. 
Mr. L. Wren, of Lowestoft, constructed an incubator which in several hands gave excellent 
results. He employed an egg-drawer with a perforated zinc bottom, in which the eggs were laid 
on chaff, while underneath is a tray of earth, which being in water constantly exhales damp. 
He employs gas only as the source of heat, and his regulator consists of a very small brass 
tube inside another of glass considerably larger, placed in a perpendicular position. The 
smaller tube, which screws through a brass cap on the outer tube, and can thus be adjusted, 
is cut slanting at the bottom end, which reaches about half-way down the glass tube ; and 
the bottom of the latter communicates with iron pipes reaching to and traversing the egg- 
drawer. Mercury is poured into the glass tube till it fills these pipes and reaches the slanting 
orifice of the inner tube. By its expansion it rises in the glass tube and thus contracts the 
sloping orifice, and so also contracts the supply of gas, which enters the regulator through 
the inner tube and leaves by a pipe from the glass tube. Several of these incubators have 
been made, and have given good hatches, but need very careful attention. 
We come now to the last noteworthy incubator of the old school, which in some hands 
produced a very high average of chickens, and which is distinguished by one of the most sensi- 
tive and accurate regulators known. It is the invention of Mr. Henry Boyle, of Ambleside, West- 
moreland. The general appearance of the incubator is shown in Fig. 34, the situation of the small 
“ mothers” for newly-hatched chicks being here seen, also of the various parts, and of the receptacle 
