88 PACIFIC STATES FLORAL CONGEESS. 



point than when the air is stagnant. It is a good plan to light small 

 straw fires and study the movement of the smoke. Dry air at a tem- 

 perature of 32 degrees weighs 563 grains per cubic foot. The vapor 

 of water at 32 degrees with a saturation of 100 per cent weighs 2.1 

 grains per cubic foot. Air at a temperature of 25 degrees Fahrenheit 

 weighs 572 grains per cubic foot. It is plain, then, that in time, pro- 

 vided there is little motion of the air, the heavier, colder layers will 

 settle to the bottom. The loss of heat by radiation from the plant sur- 

 faces and the ground is very rapid, and with a drop in temperature, say 

 from 32 degrees to 25 degrees, each cubic foot of air has increased in 

 weight about 9 grains. This explains why we often notice the ground 

 covered with frost while thermometers six feet above the ground will 

 record 34 degrees or 36 degrees, and at a height of ten or twelve feet 

 above the ground will indicate a temperature of sometimes 40 degrees. 

 Vegetables and flowers, therefore, unless grown upon sloping or ter- 

 raced ground, are at a decided disadvantage compared with tree fruit 

 in the matter of frosts. 



NATURE OF FROST. 



It can not be too clearly emphasized that it is the low temperature 

 and not the solidification of the water which is to be guarded against. If 

 there be very little vapor in the air there may be but a slight amount of 

 frost apparent, and yet the temperature be so low as to do great injury. 

 The so-called hard, dry frosts, also called black frosts, do, as is well 

 known, much more injury than the heavy frosts. Water vapor at 25 

 degrees, completely saturated, weighs 1.6 grains per cubic foot. In the 

 fall from 32 degrees to 25 degrees, nearly half a grain per cubic foot (if 

 the saturation were 100 per cent) would be condensed, appearing in 

 visible form as a frost flake. A certain amount of heat was oriven off 

 in the transformation of this invisible water vapor into ice, and an 

 exactly equal amount of heat (known as the latent heat of vaporiza- 

 tion) will be in turn required to change this frost flake back into 

 vapor. We give special attention to this point because it would appear 

 theoretically that the secret of successful protection of garden truck and 

 delicate flowers will be found in this action of water, both in setting 

 free heat at the time when the temperature is falling, and on the other 

 hand in using up heat and thus acting as a retard or brake when the 

 temperature begins to rise quickly. 



It is now quite generally believed that as much injury results from 

 the sudden warming up of the dormant and thoroughly "chilled flower 

 or vegetable as from the chilling itself. In the work of protecting 

 fruits from frost, it has been found very necessary to interpose some 

 screen early in the morning, between the sun's ravs and the frosted 



