-1859] WRITINGS OP JOSEPH HENRY. 265 



If in accordance with the foregoing an upward motion 

 takes place from any cause whatever in a mass of air satu- 

 rated with vapor, a precipitation must instantly follow. For 

 example, if we suppose the moist air to be raised to the 

 height of 1,000 yards, and if we further suppose the tem- 

 perature at the surface to be 70° the temperature at the 

 height of 1,000 yards will be 60° ; and if we inspect table B 

 (page 225), at these numbers, we shall find opposite 70° 

 7"99 grains of vapor for each cubic foot; and opposite 60°, 

 5"75 grains of vapor for each cubic foot. In this case there- 

 fore nearly 2"24 grains of vapor will be converted into water 

 and fall as rain. We see from this simple consideration 

 that the mere upward motion of a portion of saturated air, 

 from whatever cause produced, must give rise to a precipita- 

 tion of vapor in the form of water. It may not be in suf- 

 ficient quantity to come to the earth in the form of rain, but 

 may remain in the air in the intermediate state of a fog or 

 a cloud. 



If the air be not saturated entirely with vapor no precipi- 

 tation will ensue until it rise to the height at which it be- 

 comes by the diminution of temperature fully saturated. 

 Suppose for example the air at the surface is 70°, and the 

 vapor in it is that due to 65° ; then it is plain that it must be 

 reduced in temperature 5° before precipitation commences, 

 and this reduction will take place at the height of 500 yards, 

 since, as we have just stated, the reduction of temperature is 

 one degree for each 100 yards of ascent. And by this simple 

 method Mr. Espy has shown that we may, on a given day, 

 approximately estimate tlie height of the base of a cloud by 

 merely knowing the dew point at the surface of the earth; 

 for if we find that while the temperature of the air is 70°, 

 there is required at the same time to produce a deposi- 

 tion of dew on the exterior surface of a tumbler, a reduction 

 of temperature of 6° (for example) of the water within, the 

 cloud would be 600 yards above tlie surface of the earth, 

 because it will be necessary that the vapor should rise to that 

 height in order that the whole mass may be cooled to the point 

 of deposition. The bottom of this cloud will be horizontal, 



