[Illustration: FIG. 9.—Continuous records of the temperature 5 feet and 35 feet above ground on a tower in a pear orchard. Note the large difference in temperature at the two levels before the orchard heaters were lighted at 4 A.M. By 5 A.M. the temperature was practically the same at the two levels, showing that the heat from the burning oil had been nearly all expended in raising the temperature of the air within 35 feet of the ground. This point is further illustrated by the fact that at 5 A.M. when most of the heaters were extinguished, the temperature at the 5-foot level fell rapidly, while it remained practically stationary at the 35-foot level.—_Weather Bureau._] In many orchards in the Rocky Mountain States, where fruit growing is highly profitable and the injury from frost more than probable every year, an extensive use is made of oil and other fuel-burning heaters between the rows of trees. Those who wish further information with regard to this matter should send to the Weather Bureau, Washington, D. C., for Farmers’ Bulletin No. 1096. At first thought it would seem that heat so applied would be blown away or instantly escape upward. But on frosty nights there is not much wind; if there is, there is little danger from frost. And then, as previously stated, on such nights there is what is called temperature inversion, and the temperature actually rises with the first few feet of ascent, and the heated air soon reaches air of its own temperature, when no further ascent occurs. When the air forty feet from the ground is ten degrees warmer than it is around and in contact with vegetation, as often occurs on frosty nights, the heat from the fires is nearly all expended in raising the temperature of the air within this forty feet. Figure 9 furnishes the result of an experiment illustrating the correctness of the foregoing theory. [Illustration: FIG. 10.—Average dates of last killing frost in spring.] [Illustration: FIG. 11.—Average dates of first killing frost in fall.] Figures 10 and 11 show the average dates of the last killing frost in spring, and of the first killing frost in fall. CHAPTER VIII WIND AND PRESSURE OF THE GLOBE CAUSE OF LOCAL WINDS AND OF GENERAL CIRCULATION =General Circulation.= Differences in temperature, changing the specific gravity of the air, are the cause of the general circulation of the atmosphere about the earth, modified by the rotation of the earth; likewise the local circulation between land and water is caused by the different quantities of heat radiated by the two widely differing forms of matter, each attaining to a different temperature under the influence of the same solar radiation; and the inflow of winds to the cyclone and the outflow from the anti-cyclone are due to the same forces that cause the general and the local circulations.