76 



THE BOOK OF POULTRY. 



from hot flues or a reservoir in the upper part 

 of the egg--chamber. In the other case a tank 

 of water is heated by means of flues passing 

 through it, or part of the tank itself extends 

 outside the incubator case to a boiler round the 

 lamp flame by means of which a circulation is 

 set up of heated water from, and cooler water 

 to, the boiler. Radiation downwards from the 

 tank heats the air in the egg-chamber. Both 

 methods are therefore in effect the same, and 

 both are really hot-air incubators. 



" There are, however, important differences 

 in the method of ventilation and in maintain- 

 ing humidity in the egg-chamber. In incuba- 

 tors with hot-water tanks filling the upper part 

 of the case the cold air enters and rises from 

 below, and is heated by radiation as it rises. 

 As the temperature of the egg-chamber is kept 

 stationary at 104° by the rise or fall of the tem- 

 perature of the tank, the volume of air passing 

 through the fixed inlets and outlets varies with 

 the temperature and density of the outer air ; 

 the colder the latter is the more air passes 

 through the incubator, and the warmer the less 

 air. For instance, if the outer temperature were 

 to rise so high as 104°, there would be no 

 change or passage of air through the incubator 

 at all, as the air inside would be of the same 

 density (being of the same temperature) as the 

 air outside. On the contrary, at freezing point, 

 say, the very wide difference between the inside 

 temperature of 104° and the outside tempera- 

 ture of 32° and the consequent density of the 

 latter induce a very rapid change of air which 

 dries the eggs, too rapidly in the latter case and 

 too little in the former case. 



" In hot-air machines the fresh air goes in at 

 the top at one uniform high temperature regard- 

 less of the outer temperature ; the cooler this is 

 the more heat is absorbed in the heater to raise 

 the ingoing air to the required temperature. 

 The heated air gradually descends to the lower 

 and also unvarying temperature about the eggs, 

 below which it diffuses outwards at a rate, like 

 the case of the tank machine, in proportion to 

 the difference in temperature (and density), the 

 area of outlet remaining the same, between the 

 inner and outer air, the colder and denser the 

 latter the more rapid the diffusion. But in these 

 machines the area of outlet can be increased by 

 removing the bottom felt or felts, or opening 

 the incubator bottom, thus making up by in- 

 creasing the diffusive outlet area what is lost by 

 the reduced sfeed of diffusion as the outer and 

 inner air get nearer the same density. With the 

 fixed outlet ventilating holes above the eggs in 

 tank incubators the outlet cannot be increased 

 for hot weather. The number of holes should, 



therefore, be sufficient for warm weather and 

 some of them corked up for cold weather. This 

 could be effected by having slides working on 

 the ' hit and miss ' principle to control the holes 

 or area of outlets, which, in this case, could be 

 much larger to provide for warm weather. 

 There is little, if any, difference between the 

 two types as regards even distribution of heat. 

 The mobility of the volume of cooling air, 

 gradually descending, is such as to render the 

 temperature uniform at any given level. The 

 water in a tank-machine has an even tempera- 

 ture at the bottom of the tank, unless this even- 

 ness has been interfered with by deposits of 

 sediment, but in the imparting of this heat by 

 radiation through the intervening two to four 

 inches down to the eggs there is as great a likeli- 

 hood of inequalities arising, especially as the 

 ventilation is from below, as with descending 

 air. 



" Air heats and cools much more quickly 

 than water, therefore should the supply of arti- 

 ficial heat fail, either wholly or partially, a hot- 

 air incubator would much sooner fall to the 

 temperature of the outside atmosphere than 

 would a tank-machine, as the latent heat in the 

 water would keep the temperature in the egg- 

 chamber from falling injuriously for many 

 hours. For the same reason heating by hot 

 water tends to prevent fluctuations and to check 

 sudden accidental rises of temperature. 



" In the tank incubators the moisture is in 

 the bottom. The ingoing air becomes partially 

 charged at a low temperature, but as it rises and 

 becomes hotter its capacity for absorption of 

 moisture increases, and thus there may still be 

 a tendency to dry the eggs too much, especiall)' 

 if coupled with a rapid passage of air, as in 

 cold weather. In hot-air incubators the heated 

 air goes in at the top at an unvarying high tem- 

 perature. As it descends and cools its capacity 

 for absorption is reduced (which is one of the 

 reasons why no applied moisture is necessary) 

 and therefore passes the eggs with less danger 

 of drying them. 



" Carbon dioxide and other gases exhaled 

 from the eggs during incubation are heavier 

 than pure air and more readily fall with de- 

 scending ventilation than rise with the ascend- 

 ing air in tank machines. 



" Hot-water tanks are liable to give trouble 

 from leaking after being in use for a time, and 

 when that happens they are troublesome to re- 

 pair. There are also more interval flues in a 

 tank machine, which increases the risk of 

 trouble from getting choked up with soot, which 

 is often a cause of failure to get the heat up 

 sufficiently quickly." 



