468 



Scieniific Intelligence. 



[Jims, 



Evaporation is Jilways proportlcnal to the extent of humid surface. 

 Tlie quaonty of evaporation from a wet ball is the same as from 

 an equal plane surface^ or from a circle having twice the diameter 

 ' of the sphere. In the atmometer the humidity transudes through 

 the porous surface, just as fast as it evaporates from the external 

 surface; and this waste is measured by the corresponding descent 

 of the water in the stem. At the same time the tightness of the 

 collar taking off the pressure of the column of liquid, prevents 

 it from oozing so profusely as to drop from the bail ; an incon- 

 venience v/hich in the case of very feeble evaporation might 

 otherwise take place. As the process goes on a corresponding 

 portion of air is likewise imbibed by the moisture on the outside; 

 and being introduced into the ball rises in a small stream, to 

 occupy the space deserted by the subsiding of the water in the 

 tube. The rate of evaporation is not affected by the quality of 

 the porous bail, and continues exactly the same when the 

 exhaling surface appears almost dry, as when it glistens with 

 abundant moisture. The exterior watery film attracts moisture 

 from the internal mass with a force inversely as its thickness, and 

 will therefore accommodate the supply to any given degree of 

 expenditure. When this consumption is excessive the water 

 may be allowed to percolate by unscrewing the cap, avoiding, 

 however, the risk of letting it drop from the ball. 



V. Hygrometer. 



None of our readers who have paid any attention to meteoro- 

 logy can be ignorant of the great importance of knowing the 

 quantity of moisture that exists in the atmosphere at any time, 

 and of the numerous contrivances invented for that purpose 

 under the name of hygrometers. Of all these there are none 

 that can be accurately compared with each other except the 

 hygrometer of Mr. Leslie, which is merely his differential ther- 

 mometer having one of the balls covered with a piece of silk. 

 The silk being kept moist, cools the ball by its evaporation ; and 

 the degree of cold being proportional to the rapidity of evapora- 

 tion, that rapidity is marked by the sinking of the liquid in the 

 other tube of the thermometer. Hence it appears that his 

 hygrometer is similar in its indication to his atmometer, indi- 

 cating only the rate of evaporation. But there can he no doubt 

 that the knowledge of this rate and of the temperature is, suffi- 

 cient to make us acquainted with the quantity of vapour in the 

 atmosphere at the time the observation was made. 



Mr. Leslie has lately invented another kind of hygrometer, 

 which, though imperfect in its principle, may, however, be of 

 some utility, and deserves therefore to be known. On that 

 account it will be worth while to give a description of it here, 

 lie gives it the name of hygroscope. A piece of nne-grain-ed 



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