104 The Weather. 



a continual process of vertical and horizontal distillation, and 

 the beneficial rays of the sun would thus be excluded. But 

 the pressure of the vapour is very small (seldom exceeding an 

 inch of mercury at the equator), and the powerful impulses of 

 the wind are sufficient to convey to every part of the globe the 

 salutary changes of weather which it requires. Thus it happens 

 that dry air and fine weather are everywhere imported from the 

 polar quarter of the globe, and rain and cloud from the oppo- 

 site direction by the wind. 



The atmosphere is accordingly far from being everywhere 

 so saturated with vapour as if it were at rest. To determine 

 the proportion of its moisture to that of complete saturation, 

 or, in other words, the relative humidity of the air, at all times 

 and places, is the special object of hygrometry. Different 

 instruments called hygrometers {vypos, moist; fierpov, a measure) 

 have been invented for this purpose, but with only partial 

 success. Daniell's hygrometer (or different forms of this 

 hygrometer), and Mason's hygrometer, or the wet and dry bulb 

 thermometers, yield the most satisfactory results. The first of 

 these instruments fixes at once the " dew-point " of the air, or 

 the temperature at which its moisture begins to be deposited 

 in the form of dew. This is the temperature to which air must 

 be reduced in order to saturate it completely with the vapour 

 it contains. The pressure (or amount) of its vapour is then 

 easily ascertained from a table of saturating pressures, as well 

 as the additional amount necessary to complete the saturation 

 of the air. Mason's hygrometer consists of two ordinary ther- 

 mometers, the bulb of one of which is kept constantly wet, 

 and shows the " temperature of evaporation." The bulb of the 

 other thermometer is dry, and registers the temperature of the 

 air. From the readings of this instrument a set of hygro- 

 metrical tables were constructed by Mr. Glaisher, which afford, 

 under all circumstances, the required conditions of the air. 

 These are — 1st, its dew-point; 2nd, the pressure of its vapour ; 

 3rd, its relative humidity (before described) ; 4th, the weight 

 of water contained in a cubic foot of the air ; and 5th, the 

 additional weight of water required to saturate a cubic foot. 



It has been remarked by Eedfield, one of the first meteoro- 

 logists in America, that very brisk winds are often limited 

 vertically to very thin sheets or layers of the air. " Few facts/' 

 he writes, "in meteorology are more worthy of our attention 

 than the stratiform character and vast horizontal extension of 

 the aerial currents in different parts of the globe.-" This cha- 

 racteristic property of the atmosphere, by which its extensive 

 ventilating system is rendered complete, undoubtedly forms 

 one of the most striking features in meteorology. In describ- 

 ing the vast circulation of the trade winds, the consideration of 



