84 



THE BOOK OF POULTRY. 



better thermostats, or by adjusting these accord- 

 ing to the outside barometer and thermometer. 

 So also, ventilation through free apertures 

 generally so dried up the eggs that ample 

 moisture was necessary ; and the cold bottom 

 supply in so many tank machines very generally 

 strikes a rough balance, and gives success. But 

 there are also many failures ; for no such plan 

 gives the same moisture always, and it is not 

 correct to state, as in a treatise before us, that 

 Dalton's tables of water-vapour tension imply 

 any such consequence as that the moisture of a 

 chamber over cold water will always be the 

 same. The temperature is not always the same 

 over the water-trays in the machine described ; 

 and the rate of movement (and consequent titne 

 allowed any given portion of air to take up 

 vapour) is all-important, and differs immensely 

 in various states of the atmosphere. When the 

 men v/ho ran fifty large incubators at a time, 

 and the experts who manufactured machines for 

 them, began to study thequestion systematically, 

 they soon remembered that there was no draught 

 and no moisture at all in the old egg-ovens ; and 

 that the plumage of the hen, over the concavity 

 of her nest, prevented any but very slow inter- 

 change of air there, while she also was able to 

 hatch eggs upon a dry shelf, as well as upon 

 damp ground. They concluded that Nature 

 had supplied enough moisture within the &^^ 

 itself to hatch it successfully ; and they were 

 right in the main, though some of them have 

 perhaps carried that principle a little to excess. 

 They therefore began to slow tlie ventilation ; 

 not to stop it, but to check actual current of air 

 as much as possible, remembering also that 

 twice a day the drawers would be opened, and 

 the air renewed that way. Ventilation from top 

 to bottom, which is slower in movement than 

 from bottom to top, became the rule ; and that 

 was further hindered in actual flow by circuitous 

 flues, or perforated or porous distributors, till 

 the climax v,-as perhaps reached in porous walls 

 forming the sole medium for exchange. At 

 once moisture became unnecessary, and many 

 incubator users rushed to the conclusion that 

 none was ever needed, even in well-known 

 machines of a somewhat older school. Fre- 

 quently that, too, proved to be the case, and 

 many people began to get better hatches by 

 quite abandoning moisture, even with hot-air 

 machines, often stated in England to require 

 " more moisture " than the tank machines. And 

 wheri that was not quite the case, results were 

 obtained which were in some cases startling. 

 Nevertheless, many people would no doubt do 

 much towards more successful hatches by filling 

 the apertures for ventilation with loose cotton 



wool, so as to allow interchange of air while 

 stopping current, both at top and bottom 

 of machine ; and then using no moisture for 

 sixteen or seventeen days, and only a little at 

 the close, using more in dry v/eather. 



Such, however, is rather a blind guess ; and 

 in the United States they have now reduced 

 the whole matter to a science. It is 

 all determined by the size of tli-e air- 

 cell in the developing egg. This was 

 first ascertained with hen-hatched 

 eggs, and diligently compared with incubator 

 eggs more or less successful ; with the result that 

 while different eggs would vary a little in the 

 same hatch, on an average the air-cell should 

 show enlargement at the fifth, tenth, fifteenth, 

 and eighteenth daj's, about as in the diagram, 

 Fig. 50. After the nineteenth day, the beak of 



The Air-cell 

 as a Test. 



Fig. 50. — Air-cell. 



the chicken may at any time pierce the mem- 

 brane, and the head burst into the cavity, when 

 comparison is impossible. 



It should be clearly understood that sufS- 

 cient evaporation to thus enlarge the air-cell is 

 necessary to successful hatching, and that too 

 much moisture acts by preventing this. The 

 tissues of the chick are then too soft and moist, 

 and the egg is packed too full at the last for the 

 chick to turn round and break its shell. On the 

 other hand, too large a cell means too small a 

 chick, and too tough and leathery a membrane, 

 and perhaps actual glueing of the chick to the 

 shell. If the air-cell is allowed to dry out too 

 much, the chick can be swelled afterwards by 

 giving more moisture ; but this does not really 

 repair the mischief; it will never be so strong a 

 chick as if all had gone right. In the majorit\- 

 of American incubators, best average results art- 

 said to be obtained by giving no moisture foi 

 about seventeen days ; then a little or not. 

 according to the size of the air-cell. In our 

 moister climate the same might hold even with 

 more ventilation, but for the strong up-draught 

 of many machines. The breeder who examines 

 the air-cells by his tester, however, and notes the 



