MANUAL OF AGKICL'LTURE. 255; 



through the successive stages of embryonic growth, up to the 

 stage when it can maintain its separate existence as a fully- 

 developed animal of the species ; whereupon the union is broken 

 and the young animal expelled from the mother's body by a won- 

 derful muscular action of the uterus. Until able to provide for 

 itself, the young animal is fed with the mother's milk, elaborated 

 from the blood by her mammary glands. The testes in the male 

 animal are the corresponding organs to the ovaries; they secrete 

 the seminal fluid, containing sperm cells, — spermatozoids, — wdiich 

 being discharged during the sexual act, and coming into contact 

 with the ova, serves to fertilise the latter. By castration, or the 

 removal of the testes, the male animal is rendered inca- 

 pable of performing his share of the reproductive process, butat 

 the same time he becomes more docile in disposition, and he is 

 more easily and economically fattened than he could have been 

 before the deprivation. The female when deprived of the 

 ovaries assumes the like characteristics. In the case of fowls 

 the ova, whether fertilised or not, are expelled from the body in 

 a complete state in the form of eggs. They contain within the 

 shell a thick layer of nutritious albuminous matter, — the " white," 

 — which affords nourishment, when impregnation has been 

 effected, to the growing embryo during the period of incubation. 

 The physiological functions of correlation hardly fall within 

 the scope of such a work as the present. 



Oh AFTER VI. — Of Meteorology. 



The atmosphere, or aerial rind enclosing our globe, is composed 

 of a mixture of oxygen and nitrogen, and a small proportion 

 therein of one or two other gases. Purified air consists of 4 

 volumes or 77 parts by w-eight of nitrogen, and 1 volume or 23 

 parts by weight of oxygen. But common air also contains a 

 considerable quantity of watery vapour, and carbonic acid as 

 well, the latter in the proportion of about 4 volumes to 10,000 

 volumes of air ; and 1 part of anmionia to 1 million parts of air. 

 Nitric acid is also present, but in minuter quantity than am- 

 monia. The height to which it continues above sea -level is 

 uncertain, and variously estimated at from 45 miles upwards. 

 At the sea-level its average pressure upon all objects is at the 

 rate of 15 lbs. on every square inch, and it can support a column 

 of mercury 30 inches high in a tube in vacuo, whose only open end 

 enters tlie mercury contained in an open vessel. Such an arrange- 

 ment constitutes the invalual)h; instrument, the barometer, the 

 measurer of atmospheric pressure, and as sucli, an indeiinite 

 multiplier of science. Aerial density gradually diminishes as we 

 ascend above the sea-level, and with it aerial pressure as well. 

 Heat expands air, which decreasing in density, ascends ; its 



