VARIOUS INCUBATORS. 63 



heat " of water makes a large body of it like this very much 

 more " steady " in temperature than tanks of less content. 

 But much more than this, the construction of the tank was 

 found to be peculiar ; and was, in fact, the great excellence 

 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 

 very 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 partitions, which compel 

 the hot water to take a roundabout course. 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. Thus the heat percolated very slowly downwards, 

 and while the water drawn off (from three to six gallons) is 

 generally about 146, and replaced by water at 212, 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. The heat was also given to the eggs from above, 

 but this had been done in many previous machines. 



For a year or two attention was confined to minor im- 

 provements in this " 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 



