These incubators are hot-air machines, and the hot-air chamber is situated above the egg-drawer and is traversed by several flues opening out from a main one. The temperature regulating apparatus appears to be similar to that of Hearson’s machine and operates by a thermostat, which through the agency of levers opens or closes a valve over the main flue.
The Westmeria incubators (Leighton Buzzard) are of two patterns. One type is built on the hot-air principle and the other on the hot-water system. In both forms the heated air from the heating surfaces is deflected down on the eggs and escapes through the perforated bottom of the egg-drawer. The inlet air is first warmed by contact with the main flue. The thermostat is similar to that in the Hillier machine (fig. 7) and consists of a coil mounted on an axis, round which it can rotate. The coil is filled with mercury and is closed at one end. Between this end and the mercury column is a short column of air. By expansion of the air under a rising temperature, the mercury column is displaced and brings about a rotation of the disk to which the coiled tube containing it is attached. This rotation raises the cap over the main flue.
All the incubators so far described have been constructed with the idea of obtaining as nearly as possible a uniform temperature. But in E. S. Renwick’s incubator (America) no attempt is made to obtain uniformity in temperature. On the other hand, it is designed to give a periodical oscillation from one extreme to the other of a limited range, about 3°, of temperature. This is accomplished by means of a thermostatic bar made of plates of brass and vulcanite fastened together. This is connected with a clockwork and detent arrangement, which simultaneously opens a valve and actuates the lamp flame. The temperature falls to the lower limit of its range before the thermostatic bar is sufficiently bent to set the clockwork arrangement operating in the reverse direction, by which the valve is closed and the lamp flame increased. The temperature then rises to the higher limit, when the bending of the thermostatic bar again releases the detent and the clockwork opens the valve and reduces the flame.
The incubator is said to succeed well. It also possesses a mechanical arrangement by which all the eggs can be periodically turned on rollers at once.
Size.—The incubators which have been described are of relatively small size, and the numbers of eggs which they can incubate are strictly limited. For commercial purposes, however, operations of a much larger magnitude are desirable and necessary. And there can be no doubt that for these purposes the incubators of the future will be of great size and will contain from 15,000 to 30,000 eggs or more at a time. Already, at Aratoma Farm, Stamford, New York State, there is established a large incubation room, containing several thousands of eggs, and in which the heat regulation is controlled in part by the personal efforts of attendants. It constitutes almost a complete return, with added accessories, to the methods of the Egyptians, and to those of John Champion.
Bacteriological Incubators.
These differ from bird Incubators in that the heating surface of the incubation chamber generally surrounds all sides of it and there is, as a rule, no special arrangement for bringing about a more or less humid condition of the contained air. In some forms there is an arrangement to ensure a continuous supply of fresh and moist air, but in the majority the incubation chamber obtains its supply of fresh air vicariously. In some forms the chamber of the incubator is heated by a warm water tank of a simple kind, which extends round all its sides. But in other forms a series of tubes or flues passes through the water in this tank and thus simulates in principle the tube boiler. This latter form utilizes the heat of the flame to a greater degree than the former kind. In yet other forms the incubation chamber is heated by warm air chambers which surround it or flues which traverse it. Most bacteriological incubators are square or rectangular in form, but some bacteriologists prefer cylindrical forms, presumably on account of the ratio of volume to surface in connexion with the water tank.
One of the best known and most generally used of the cylindrical and water-tank kind is that of Dr d’Arsonval. It consists of two copper cylinders (fig. 8 C and C′), each terminating in a cone below. Between the cylinders is a wide interspace, in order that a large volume of water may be contained. This interspace therefore constitutes the water-tank of the incubator. The upper orifice of the inner cylinder is closed by a movable double lid, which contains an interspace filled with water. The outer cylinder has an oblique form at its upper end and is permanently closed. The result attained by this slope of the lid of the outer cylinder is that the water tank, which is fed from the highest point, becomes completely filled. The aperture at the highest point of the outer cylinder is plugged with a caoutchouc plug and through a perforation in this a glass tube (T) is placed. In the side of the outer cylinder below this, there is a wide and rimmed aperture, to which a gas regulator of special construction is fixed.
| Fig. 8.—D’Arsonval Incubator. |
This regulator was designed by Théophile Schloesing, and consists of a brass box, supplied with a rim (L) which fits on to the corresponding rim (L′) on the aperture of the incubator. Stretching across the orifice thus connecting the brass box of the regulator with the water-tank of the incubator is a thin india-rubber diaphragm (D). At its outer end a perforated cap (R) screws on to the brass box. Through the perforation the inlet gas tube passes (I); the outlet gas tube (O) leaves the brass box below and passes direct to the gas burners. The inlet gas tube is fitted at its inner end with a sliding flanged collar (F), which is kept pressed against the rubber diaphragm by a spiral spring. Just behind the collar the inlet tube is perforated by a small hole, so that the gas supply is never wholly cut off, even though the rubber diaphragm completely occludes the inner aperture of the pipe.