Among the plant formations and associations of the lower grounds which we considered in Chapter II. it was noted that the controlling factors were mainly connected with the nature of the soil and the amount of the water-supply. Here on the mountains another factor, the climatic, comes in emphatically, and takes charge. The temperature of the atmosphere falls one degree centigrade for about every 200 feet of elevation, so that a sharp frost on the lowlands may easily mean zero Fahrenheit on a 4,000-foot hill. The rarefaction of the atmosphere, too, tends to produce a much greater range of temperature, both diurnal and seasonal. Again, the velocity of the wind is much higher on the summits than on the plains, where friction is greatly increased by trees and other obstacles. These high winds have a very great cooling effect, as we may notice on our own bodies even in summer. In fact, as regards climatic change, an ascent of a thousand feet is comparable to a journey of several hundred miles northward. Anyone who has, on a winter tramp, been caught in a snowstorm on a 3,000-foot hill is forcibly reminded of what he has read of winter conditions in the Arctic regions. In ascending Ben Nevis we travel, in a sense, to the Arctic Circle. But the analogy is false, for conditions, especially in summer, are very different in the two places. The plants of our mountains have all the advantages of the high summer elevation of the sun, very different from the weak, sloping sunlight of the [Illustration: FIG. 32.--ALPINE PLANT-BOSS (_SILENE ACAULIS_, _HYMENOPHYLLUM UNILATERALE_, _MNIUM HORNUM_). [_To face p. 191._] Arctic. On our loftier hills, indeed, the heat is on occasions oppressive.