INDICATORS OF WEATHER. 153 



scend through the atmosphere at a uniform rate ; and this rate of 

 motion is not in g-eneral so great as to cause injury when they 

 reach the earth. 



Question. I woukl like to inquire of the Professor if the indi- 

 cations of the barometer are sufficiently accurate as to make it 

 advisable for a farmer to depend upon them ? 



Prof. Fernald. The indications of the barometer are doubtless 

 very much more reliable upon the ocean than upon the land, since 

 the winds are so variable upon the land. Then the fact of eleva- 

 tion comes in .to modify the herght -of the barometer, so that the 

 ordinary reading of a barometer, "fair," or "rainy," etc., amount 

 to nothing, unless it is adapted to the elevation at which the bar- 

 ometer is used. Ordinarily the barometer cannot be relied on as 

 an accurate guide as regards prospective rainfall, and the meteor- 

 logist depends quite as much upon facts furnished by the hygrom- 

 eter, an instrument vrhich is used to indicate the amount of 

 moisture in the air. There is a simple instrument called the 

 hj'grodeik, which costs, perhaps, not more than eight or ten dol- 

 lars, which any one can use to determine the condition of the air. 

 This hygrodeik, or hygrometer, has a wet and a dry bulb, the latter 

 of which indicates the temperature of the atmosphere. The wet 

 bulb is covered with muslin, which is kept constantly wet. When 

 the atmosphere is in a dry condition, there is a difference of a 

 number of degrees between the indications of the wet and dry 

 bulb. From what cause ? The wet bulb, being covered with 

 cloth, which is wet, evaporation takes place from it, and when the 

 atmosphere is dry this process goes on rapidly, and the difference 

 between the two temperatures is considerable, ranging from zero 

 to 20 or more degrees. With data furnished by this instrument, 

 using tables which have been computed for this purpose, one can 

 determine the exact per centage of moisture in the air. The drier 

 the air is, tlie greater will be the difference between the reading 

 of the dry and wet bulb. If the observer finds the difference is 

 very slight ; if for instance one is T0° and the other 69°, he would 

 know that the air is charged with perhaps 95 per cent, of 

 all the moisture it is capable of containing, and would be likely 

 to expect, especially if his barometer were falling, that there would 

 be rain very soon. The hygrometer may be depended upon more 

 than the barometer, and the two together may serve as very good 

 indicators of the weather. Rain seldom falls until the dry bulb 

 and the wet bulb exhibit the same height of mercury, when the 



