306 Domestic Economy. 



outside of the box afTord but little surface to be cooled by the surrounding 

 air, so the force of the circulation, which is always in the ratio of the differ- 

 ence between the temperatures of the waters passing out of the calorifcre 

 and reentering it, does not become greatly diminished, even after having 

 expended a large portion of its heat, on the outside of the box, in main- 

 taining a gentle heat in the cnge (c) adjoining to it. 



We see, therefore, that the more the water is cooled which passes 

 through the last circumvolutions of the tubes, the more active is the cir- 

 culation in all parts ; and, consequently, the more equal is the temperature 

 of all the tubes which heat the box, and of the air within it : indeed, to 

 prevent the loss of heat as much as possible, the boiler, and all those parts 

 of the tubes which are placed on the exterior of the box, are enveloped in 

 lists of woollen cloth. M. Bonnemain having thus applied these principles 

 with so much skill, is always enabled to maintain in these boxes an equal 

 temperature, varying scarcely so much as half a degree of Reaumur's ther- 

 mometer ; but, as if it was not sufficient to have thus far resolved the 

 problem, he contrived that this degree of temperature in all parts of the 

 stove should be constantly maintained at that point which was found most 

 favourable for promoting incubation. It was by means of an apparatus for 

 regulating the fire, that he attained this desirable object. 



The action of this regulator is founded on the unequal dilatation of dif- 

 ferent metals by heat. A movement is communicated near to the axis of a 

 balanced lever, which lever transmits it b}' an iron wire to a register in the 

 ash-pit door of the furnace. Combustion is, by this means, abated or in- 

 creased. The details of this piece of machinery are fully described and 

 delineated in Gill's Technological Re'po&itory (Feb. 1828, p. 70.); but a 

 much more complete regulator would be the thermometer of Kewley, which 

 we have so often commended to little purpose. 



When we would hatch chickens, we light the fire and raise the tem- 

 perature till we obtain that degree of heat in the box which is fitted for 

 incubation ; we then place the eggs near to each other upon the shelves, 

 with borders to them (j, i), which are fixed under each row of tubes. Tt is 

 convenient not to cover, on the first day, more than the twentieth part of 

 the superficies of the shelves, and to add every day, for twenty days, an 

 equal quantity of eggs ; so that, on the twenty-first day, the quantity of eggs 

 first placed will be for the greater part hatched ; so that we may obtain 

 every day nearly the same number of chickens ; but which may, never- 

 theless, be occasionally regulated by the particular season of the year. 



During the first days of incubation, whether natural or artificial, the 

 small portion of water contained within the substance of the egg evaporates 

 through the pores in its shell : this is replaced by a small quantity of air, 

 which is necessary to support the respiration of the chick ; but as the 

 atmospheric air which surrounds the eggs in the box at that degree of tem- 

 perature is either completely dry or but little humid, so the chick would 

 greatly suffer, or finally perish, from this kind of desiccation. The aqueous 

 vapour which exhales from the breathing of the old fowls whilst hatching 

 in som.e degree prevents this ill effect ; but, nevertheless, in dry seasons, 

 this vapour is hardly sufficient ; and thus, in order that the eggs may be 

 better hatched in the dry seasons, the hens cover them with the earth of 

 the floor of the granary. 



In artificial incubation, to keep the air in the stove constantly humid, 

 they place in it flat vessels, such as plates (k k), filled with water. 



When the chickens are hatched, they are removed from the stove, and 

 carried to the cage (c), where they are fed with millet, and nestle under a 

 sheep's skin, with wool on it (/), suspended over them. They also separate, 

 by means of partitions in the cage, the chickens as they are hatched each 

 da}', in order to modify their nourishnjent agreeably to their age. 



