6c 
The Illustrated Book of Poultry. 
of the invention of Messrs. Roullier and Arnoult. If we take a Florence flask of water containing 
a few particles of bran, and apply a lamp to the bottom , we shall see how the heated water rises 
and circulates, and the whole becomes hot in a very short time. But if we apply a hot plate to 
the surface of the water in an open glass vessel, there is scarcely any movement, and it is a long 
time ere the heat reaches the lower portion of the fluid. This time may be increased still further 
by horizontal septa or partitions, which compel the hot water to take a roundabout course. Now, 
the tank in the hydro-incubator was not only large, but furnished with such partitions ; and the 
boiling water was always supplied at the top. The consequence of these arrangements is, that the 
heat percolates very slowly downwards, and while the water drawn off (from three to six gallons) is 
generally about 146°, and replaced by water at 21 2°, the temperature of the bottom layer, which 
acts upon the eggs, only varies in a small degree, and that in a regular manner within certain 
limits, which appears actually beneficial to the eggs, as also discovered by Mr. Ren wick. 
It is remarkable that the all-important character of these points was at first by no means 
apparent even to the manufacturers. While to Mr. Christy is unquestionably due, as we have 
said, the introduction of a machine which reduced artificial hatching to practice, early editions 
of his pamphlet show that at first he shared the opinion, which was then generally entertained 
by all who sought to discover the reasons for that success, that this was due chiefly to the absence 
of any lamp, and freedom from noxious gases arising therefrom. Such an opinion was, in fact, 
natural enough, and for some time attention was confined to minor improvements in the original 
“ hot- water ” form of machine. The first of these was the freer supply of ventilation. Gradually 
also was arrived at the proper area of damp earth underneath the eggs to provide the proper 
amount of moisture ; these machines using, in place of cold tanks, earth baked to kill all life, 
and moistened with water on each occasion when the eggs were attended to. Still later it was 
found that during the first eight or ten days the eggs did well in a close atmosphere with little 
ventilation, whilst later on they absolutely needed fresh air ; that, as the embryos grew, the eggs 
themselves did far more in imparting heat to the machine ; and that to be putting in cold eggs 
amongst others far advanced was most injurious to the total result. These latter facts were 
largely established by the experiments of the late Mr. Alexander Comyns, Hon. Secretary to the 
Poultry Club, and they led to the provision of a smaller machine to “start” the eggs in a 
comparatively close drawer, from which they could be transferred later on to a second machine. 
Afterwards, however, it was found preferable to provide two drawers, one smaller than the 
other, in which these different conditions could be preserved in the same machine. 
Incubators worked by hot water are now made by several manufacturers as well as by Messrs. 
Christy and Co., the “ Gem ” hatcher, and several others, being modelled more or less closely 
on the French machines of Messrs. Roullier and Arnoult. By packing the tank and drawers all 
round with a good thickness of sawdust or other material to retain the heat, somewhat smaller 
tanks than at first have been made practicable, but still very large compared with those formerly 
employed, while the horizontal partitions are more or less essential. These incubators are 
made as small as for three dozen eggs, one of which size can be obtained for about thirty 
shilling's ; but the experience of many persons has proved that the size for ninety or a hundred 
eggs (as made by different manufacturers) is the most generally useful size, and, on the whole, 
gives most satisfactory results. Such a machine now contains about fifteen to twenty gallons 
of water, and the following is the mode of operation with it : — The machine should have a place 
free from strong, cold draughts, if possible. When fixed, it must be filled up entirely with 
boiling water, which is left in for twelve hours, and must then be entirely drawn off by tipping 
the machine forward and opening a tap at the bottom of the tank (this tap in ordinary work is 
