when moisture is more than ever an important factor. There is, according to some poultry authorities, reason to believe that the sitting hen secretes some oily substance which, becoming diffused over the surface of the egg, prevents or retards evaporation from within; presumably, this oil is permeable to oxygen. In nature, with the sitting hen, and in the “ Mamal ” artificial incubating establishments of the Egyptians, direct air currents do not exist, owing to the large size of the chambers, and consequently incubation can be successfully achieved without any special provision for the supply of moisture. Artificial incubation has been known to the Egyptians and the Chinese from almost time immemorial. In Egypt, at Berme on the Delta, the trade of artificial hatching is traditionally transmitted from father to son, and is consequently confined to particular families. The secrets of the process are guarded with a religious zeal, and the individuals who practise it are held under plighted word not to divulge them. It is highly probable that the process of artificial incubation as practised by the Egyptians is not so simple as it is believed to be. But as far as the structures and processes involved have been ascertained by travellers, it appears that the “ Mamal ” is a brick building, consisting of four large ovens, each of such a size that several men could be contained within it. These ovens are in pairs, in each pair one oven being above the other, on each side of a long passage, into which they open by a circular aperture, just large enough for a man to obtain access to each. The eggs are placed in the middle of the floor of the oven, and in the gutters round the sides the fire is lighted. The material for this latter, according to one account, consists of camels' dung and chopped hay, and according to another of horses' dung. The attainment of the right degree of heat is apparently reached wholly by the skill of the persons employed. ~ When this has been attained, they plug the entrance hole with coarse tow. On the tenth to twelfth days they cease to light the fires. Each “ Mamal ” may contain from 40,000 to 80,000 eggs. There are 386 “ Mamals ” in the country, which are only worked for six months of the year, and produce in that time eight broods. Many more than two-thirds of the eggs put in are successfully hatched. It is estimated that 90,000,000 eggs are annually hatched by the Bermeans.
A method of incubating that appears to have been altogether overlooked in England-or at least never to have been practised -is that carried on by the Couveurs or professional hatchers in France. They make use of hen-turkeys for the purpose, and each bird can be made to sit continuously for from three to six months. The modus operandi is as follows: a dark room which is kept at a constant temperature throughout the year contains a number of boxes, just large enough to accommodate a turkey. The bottom of the box is filled with some vegetable material, bracken, hay, heather, straw or cocoa-fibres. Each box is covered in with lattice-work wire, so arranged that the freedom of the sitting bird is limited and its escape prevented. Dummy eggs, made by emptying addled ones and filling w ith plaster of Paris, are then placed in the nest and a bird put in. At first it endeavours to escape, but after an interval of a few days it becomes quiet, and the dummy eggs being then removed, fresh ones are inserted. As soon as the chickens are hatched, they are withdrawn and fresh eggs substituted. The hen turkeys are also used successfully as foster-mothers. Each bird can adequately cover about two dozen eggs.
Incubation as an industry in Europe and America is of recent development. The growing scarcity of game birds of all kinds, coincident with the increase of population, and the introduction of the breech-loading gun, together with the marked revival of interest in fancy poultry about the year 1870, led, however, to the production of a great variety of appliances designed to render artificial incubation successful.
Previously to this, several interesting attempts had been made. As long ago as 1824, Walthew constructed an incubator designed to be used by farmers' wives with the aid of no more than ordinary household conditions. It consisted of a double-walled metal box, with several pipes opening into the walled space round the sides, bottom and top of the incubator. These pipes were connected with an ordinary kitchen boiler. Walthew, however, constructed a fire grate, with a special boiler adapted to the requirements of the incubator. Into the walled space of the incubator, steam from the kitchen boiler passed; the excess steam escaped from an aperture in the roof, and the condensed steam through one in the floor. Ventilating holes and also plugs, into which thermometers were placed, pierced the door of the incubator.
In 1827, J. H. Barlow successfully reared hens and other birds by means of steam at Drayton Green, Ealing. He constructed very large rooms and rearing houses, expending many thousands of pounds upon the work. He reared some 64,000 game birds annually. The celebrated physician Harvey, and the famous anatomist Hunter were much interested in his results. To John Champion, Berwick-on-Tweed, in 1870, belongs, however, the credit of instituting a system which, when extended, may become the system of the future, and will rival the ancient “ Mamals ” in the success of the incubation and in the largeness of the numbers of eggs incubated. He used a large room through which passed two heated flues, the eggs being placed upon a ta-ble in the centre. The flues opened out into an adjoining space. The temperature of the room was adjusted by personal supervision of the fire. This system, more elaborated and refined, is now in use in some parts of America.
B ird I incubators.
Owing to the great variety in the details of construction, it is difficult to arrange a classification of incubators which* shall include them all. They may, however, be classified in one of two ways. We may either consider the method by which they are heated or the method by which their temperature is regulated. In the former case we may, divide them into “hot-air” incubators and into “ hot-water ” or “tank ” incubators. In the latter case we may classify them according as their thermostat or temperature-regulator is actuated by a liquid expanding with rising temperature, or by solids, usually metals. In America incubators of the hot-air type with solid and metallic thermostats are most used, while in Europe the “ tank ” type, with a thermostat of expansible liquid, prevails. For the purpose of more adequately considering the various forms which have been in use, or are still used, we shall here divide them into the “ hot-air ” and “ hot-water ” (or “ tank ”) classes.
In the hot-air types the incubator chamber is heated by columns of hot air, while in the tank system this chamber is heated by a tank of warmed water.
(a) Hot-Water Incubatars.-In 1866 Colonel Stuart Wortley described in The-Field an incubator constructed upon a novel principle, but which appears never to have been adopted by breeders. The descriptive article is illustrated with a sketch. Essentially the incubator consists of four pipes which extend across the egg chamber some little distance above the eggs. The pipes pass through holes in the side of the incubator, which are furnished with pads, so as to render their passage air-tight. Externally they are connected with a boiler. This is provided with a dome through which steam escapes, and also with a glass gauge to show the height of the water within the boiler. The water in the boiler is kept at the' boiling point, and the temperature of the incubator is regu ated by adjustment of the length of the hot-water pipes within the egg chamber. To raise the temperature, a greater length of the pipes is pushed into the chamber, and to reduce it, more of their length is pulled outwards. It is claimed for this instrument that since the temperature of boiling water at any particular locality remains practically constant, the disadvantages due to fluctuations in the activity of a lamp flame or the size of a gas flame are obviated. But it has the serious disadvantage that there is no automatic adjustment to compensate for fluctuations of atmospheric temperature. And experiments by C. Hearson have shown that even if the temperature of the tank or source of heat be constant, that of the incubator drawer will nevertheless vary with fluctuations of external tem erature. Probably if the mechanical difficulties of providing a seii-regulator were overcome, it would prove an efficient and reliable incubator. The difficulties do not seem to be insuperable, and it appears possible that a thermostatic bar could be so arranged as to automatically increase or decrease the length of hot-water pipes within the incubator, and therefore the incubator temperature.
Another early form of incubator is Brindley's, which was first in