II o t- IV a ter Incub a 7 ors. 
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not used at all). The machine is then filled up with boiling water the second time. This process 
is absolutely essential to thoroughly “ charge ” the whole machine and its packing with the necessary 
heat. Twelve hours after the second filling the thermometer should be put in, and as soon as 
it falls to 106 0 (which will not be till rather later) the eggs may be placed in the drawer on 
flannel. In very frosty weather the flannel may be doubled with advantage. Also at the same 
time wet the earth-trays, and draw off from two (in warm) to three (in cold weather) gallons of 
water by the working tap, replaced by boiling water. The supply of heat must now be attended 
to every twelve hours, and about the same hour. At each visit the water drawn off will probably 
be from 136° to 140°, and must always be tested by the thermometer, as this figure is the guide 
for the quantity of boiling water to put in. But the heat of the drawers, which is also examined, 
is another guide. As a rule, if the room be about 6o°, from two and a half to three gallons may 
be required, which may rise to six gallons in cold weather in a cold room. The heat added in this 
way is very slowly and equably percolating downwards all the lime, so that the drawer varies very 
little when the quantities are chosen with judgment, while any little excess or defect on a single 
occasion has comparatively mild effects. If the machine is filled up at once, the ventilators 
should be kept nearly closed for the first nine days, half opened on the tenth day, rather more the 
next day, and thenceforth the drawer freely ventilated. If two incubators or a divided drawer 
are used, the eggs are kept in the closed locality at first, and then moved to the ventilated one. 
Particular attention must be paid to the supply of moisture beneath, and to the removal of any 
bad egg, and each time the machine is visited the eggs must be withdrawn, turned, and exposed 
to the cool air for from fifteen to twenty minutes. 
As hatching proceeds, it will be found that less and less hot water is required, owing 
to the “ vital ” heat developing in the eggs themselves. This must be carefully attended to. 
On the other hand, fresh cold eggs would lower the temperature ; and therefore fresh eggs 
added after a start should be first warmed for a minute or two in water heated to about 105 In 
a very dry room a loose pan of damp earth under the incubator is an advantage, or shallow tins 
may be placed in the egg-drawer itself to supply more moisture. The heat should be kept from 
103° to 106 0 as nearly as possible. The temperature of the drawer should be noted at a glance 
when the drawer is opened, as it will rapidly fall when exposed to the air. Eggs should always be 
tested for fertility at an early date, as bad eggs in a drawer are a great drawback to the whole 
batch ; and any fetid smell should at once lead to a rigorous examination, and the sprinkling on 
the earth-trays of a few drops of Condy’s fluid. Every two or three days, when turning the eggs, the 
outside ones should be moved to the middle, or the front ones to the back, and vice versa. Wh en 
hatching time arrives, the chicks should be removed about every twelve hours, and not oftener ; and 
if many are to be taken out, the “ vital heat ” thus abstracted must be compensated by more hot 
water than would otherwise be used. All these points are simple enough, and easily remembered 
when their reason is once clearly seen ; but in their observance lies the main secret of success with 
hot-water incubators. 
Simple as this system was, however, and in spite of the opinion at first so strongly held 
by every one, that the absence of lamps or burning gas was one of the chief reasons of the success 
of the hot-water machines, the provision of gallons of boiling water every twelve hours was found 
such a tax on most householders, that there was a demand on all hands for supplementary 
apparatus. The first and most natural step was to supply special boilers heated by paraffin oil, 
or Fletcher’s well-known gas-furnaces ; and these are still considerably used. The further step 
was however soon taken of carrying circulating pipes from a small boiler into the tank of the 
machine, and this is now the favourite and usual method of working hydro-incubators. The sole 
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