IN ITS RELA TION TO HORTICULTURE. 39 



effectual mode of sheltering any surface of ground, whether level or 

 hilly, is by planting groups or belts of trees. In this way a park or 

 pleasure-ground in the most exposed situation may be sheltered in 

 every part of its surface. In the same way, an orchard or plan- 

 tation of fruit-trees, the trees being equally distributed over the 

 ground, produces its own shelter ; but as a kitchen-garden, if planted with 

 standard fruit-trees so as to produce shelter, would be unfit for the cul- 

 ture of culinary vegetables, the best mode of sheltering it is by crossing 

 it with walls and hedges at such distances as may produce the de- 

 sired shelter in the given situations. 



The renewal and agitation of the air in plant-houses is a subject 

 which is still but imperfectly understood. It has been long 

 known in practice that plants cannot be kept in a healthy and growing 

 state in any houses that are not supplied with ample means of venti- 

 lation; and yet, admitting the external air into houses for tropical plants, 

 and for forcing fruits, is often found to be decidedly injurious. The 

 injuries sustained by the admission of the external air into a hothouse 

 are greater or less according to the difference of temperature, and, 

 consequently, of moisture. When the external air enters a hothouse 

 in which the air is at a high temperature, it rushes in with con- 

 siderable velocity, driving out by the pressure of the atmosphere the 

 hot and vaporous air by which the plants are surrounded, and 

 becoming heated and charged with moisture, at the expense of 

 the enclosed air, the earth in the pots, and the foliage of the plants. 



The best remedy for this evil is to heat the air, and to saturate it 

 with moisture, before it is admitted among the plants ; and this has 

 been done first by Mr. Penn's mode of heating, and since by the 

 Polmaise system and other modes of heating and ventilating. Both 

 systems are founded on the well-known principle that hot air is 

 lighter than cold air ; and the object sought to be obtained by both 

 is to make air circulate in the atmosphere of a plant house as water 

 does in hot-water pipes. This is done by admitting hot air at one 

 end of the house, and having an underground drain to draw off the 

 cold air at the other. Mr. Penn's plan is shown in fig. 1, in which 

 a is a chamber containing hot- 



of heating is now scarcely ever 



heard of. The Polmaise system, on the contrary, is very generally 



