74 GENERAL MANAGEMENT OF POULTRY 



certainJy no exception thought such a machine of the least 

 use for practical purposes. That when so many had devoted 

 money, pains, and complicated apparatus to keep up a regular 

 supply of heat, a machine should succeed which depended 

 altogether upon a re-supply of boiling water every twelve 

 hours, appeared to all simply ridiculous. The following year, 

 however, a competition of incubators took place at a poultry- 

 show at Hem el Hempstead, at which this incubator far out- 

 stripped all competitors ; and the success then obtained, so far 

 from being accidental or temporary, was much surpassed on 

 later occasions. " Hydro-Incubators " were sold literally by 

 hundreds, and solved the long-sought problem by making 

 artificial hatching a practical reality. 



It was some time before it was understood how it was that 

 this success had attended so apparently rude a machine. The 

 whole secret lay in two points mainly, wherein the new 

 machine differed from it predecessors. In the first place, the 

 hot-water tank was very large compared with all other apparatus 

 previously made, holding for a 100-egg machine about twenty or 

 twenty-four gallons. The enormous " specific 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 this 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 

 septa or partitions, which compel the hot water to take a 

 roundabout course. Now, the tank in the hydro-incubator 



