REFERENCE TO HORTICULTURE. 33 



suing summer, after having begun to grow ; thus showing that even the 

 hardiest of our trees may be affected, from their wood not being suffi- 

 ciently ripened in a cold wet autumn. 



The presence of a stream or river increases the tendency to slight 

 frosts in spring and autumn. The surface of the water, as it condenses 

 by cold, descends to the bottom, and a warm stratum succeeds to the 

 surface ; and so far the tendency is towards heating rather than cool- 

 ing the air : but the great evaporation that takes place through the 

 day, and early in the evening, robs the air of so much caloric, that 

 fields situated near shallow rivers, streams, or bogs, have generally 

 been found most liable to frost ; near the sea, or near great bodies of 

 deep water, the first -mentioned effect of a succession of warmer strata 

 to the surface prevails, and we have less tendency to freezing. 



Cold water applied to frozen plants early in the morning, before the 

 sun touches them, by gradually thawing them, will frequently restore 

 them, especially if they are shaded from the sun until the process of 

 thawing is complete. But no power on earth can recover the plant if 

 the juices have been exposed to freeze till the vessels are burst, which 

 may be known by the change of colour in the leaves caused by the 

 suffusion of the sap. If some of the most tender leaves only are hurt 

 on the young growths, the plant may survive ; if the wood is chiefly 

 young and succulent, as in seedlings, Dahlias, &c., the whole plant 

 generally perishes, unless where there is an old ripened root or wood 

 to renew vegetation. 



It appears from these various considerations that the best mode of 

 increasing or preserving the heat of the earth and of the atmosphere is 

 to check the loss of heat from evaporation by drainage, and from radia- 

 tion by coverings or shelter. 



Atmospheric Moisture, considered with reference to Horticulture. 



The existence of water in air, even when the latter is in its driest, 

 coldest, and purest state, is easily proved ; and the quantity of aqueous 

 vapour which it holds in suspension has been ascertained by experi- 

 ment, It varies with the temperature, increasing as the heat is greater, 

 in something like a geometrical ratio. "At 50 Fahr. air contains 

 about 1-50 of its volume of vapour; and as the specific gravity ot 

 vapour is to that of air nearly as 10 to 15, this is about 1-75 of its 

 weight. At 100, and supposing that there is a free communication 

 with water, it contains about 1-14 part in volume, or 1-21 in weight." 

 Water is also held in the atmosphere in a grosser form than that of 

 elastic vapour; for example, as mist, fog, or clouds, which three forms 

 only differ in their appearances, and not in their nature. It will be 

 found afterwards that it is of some importance to bear in mind the dis- 

 tinction between water held in suspension in the atmosphere in the 

 state of invisible elastic vapour, and water held in suspension in the 

 state of steam, mist, or fog. 



The simplest mode of measuring the amount of vapour in the air 

 is by means of the wet and dry bulb thermometer, which is the best 

 hygrometer for horticultural purposes. 



