THE F.SSF.NT1AL POINTS. 75 



was not only large, but furnished with such partitions ; and 

 the boiling water was always supplied at ttie 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 212, the temperature of the bottom layer, which acts 

 ujK)n 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. 



The all-important character of these points was at first by 

 no means apparent even to the manufacturers. 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 

 results. Hence it was found preferable to provide two 

 drawers, one smaller than the other, in which these different 

 conditions could be preserved. 



Incubators worked by hot water are now made by several 

 manufacturers, nearly all 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 



