76 MY GARDEN. 



which are useful to imbed entirely in the earth, as the warmth of 

 the earth protects delicate plants. The larger structures with loose 

 pieces of glass for vineries are, however, of questionable utility, and 



^ << FIG. 90. 



FIG. 89. FIG. 91. 



cannot be commended. For protection of plants in early spring the 

 French use cloches, or glass bells (fig. 89). In England a square iron- 

 framed glass is much employed (fig. 90), and in the West of England 

 an octangular structure of glazed zinc bars (fig. 91) is much employed 

 by market-gardeners, and has been found useful at my garden. 



Any gardener who has all the variety of frames and glass-houses 

 which we employ has only to use them with horticultural skill to 

 obtain satisfactory results. 



VENTILATION OF GLASS-HOUSES. 



The philosophy of ventilation, or change of air, in glass-houses is 

 fully considered in my garden, as the health of the plants in great 

 measure depends upon it. Firstly we rely upon the property of 

 diffusion, a power whereby one gas in contact with a second diffuses 

 itself rapidly throughout the second. An example of this may be seen in 

 a soda-water bottle, which is full of carbonic acid. This gas, although 

 much heavier than the air, diffuses itself through the air in a few hours 

 against gravitation. In a greenhouse the intervals between the pieces 

 of glass and little holes in our woodwork play an important part in 

 enabling the foul air of the glass-house to escape. 



Besides the property of diffusion, we take advantage of the difference 

 of density of hot and cold air. Hot air is light, cold air is heavy ; 

 hence by admitting cold air at the bottom of the house and warming it, 

 it becomes light, rises to the top, and escapes by any aperture. This 



