ARTIFICIAL HATCHING 



125 



the shell, owing to retarded development and want of 

 vitality. In an individual incubator the heat is regulated 

 by a thermostat and damper (Fig. 69). The action of the 

 thermostat is based upon the expansion of metals, or in some 

 thermostats on the expansion of gases. As the heat in the 

 incubator rises, the metal or gas expands, and this expansion 

 is transmitted to the damper over the heat box by means 

 of levers. If the incubator is too hot, the damper rises, and 

 allows the heat to escape. When the incubator is running 

 at too low a temperature, the metal contracts, the damper 

 falls, and more heat is thrown into the machine. 



> md fo reyufcte temperature with,, 

 /lever arm vrftich supports the damper over ffie lamp 



'Fblcrum. on vrhisJt the 

 fever arm ts balanced. 



-Damper over 

 <=> 1 tftelamp. 



Rivota folding expandin^T~~~~Attachmeni of rod 

 metal to 'non-expanding metal to thermostaL 



FIG. 69. Sketch of the parts of a self-regulating device for an incubator. 



The second requirement is moisture. Moisture prevents 

 evaporation, and also softens the membrane of the egg, 

 thereby making it easier for the chick to get out, hence a 

 relatively high degree of moisture is essential towards the 

 close of the hatch. The moisture content is read by the 

 instrument called the hygrometer. Normal ah* contains 

 about 40 to 45 per cent of moisture, or relative humidity, 

 as it is called. During the latter part of the hatch the humid- 

 ity should run as high as 58 per cent. 



Fresh air is the third of the important requisites in an 

 incubator, and to secure this in the machine some form of 

 ventilator must be used. In some incubators the bottom of 

 the machine is open or slatted; in others fresh air is carried 



