54 Meteorological Observations. [Jan. 



mented 5 or even 10 times.--In general this augmentation is 

 proportional, as in the case of cooling, to the swiftness of 

 the wind, the action of still air itself, being reckoned equal 

 to that produced by a celerity of 8 miles each hour. Hence 

 the velocity of winds is easily computed, from a comparison 

 of the indications of a hygrometer with an atmometer, or of 

 a sheltered, with those of an exposed, atmometer. Thus sup* 

 pose the hygrometer to mark 40 degrees, or the column of 

 water in a sheltered atmometer to subside at the rate of 2 

 divisions in an hour, while in one exposed to the current^ 

 the descent is 12 divisions ; then as 2 is to 10, the superadd- 

 ed effect of the wind, so is 8 to 40 miles, the distance 

 through which it has travelled in that time, 



^' The atmometer is an instrument evidently of extensive 

 application, and of great utility in practice. To ascertain 

 with accuracy and readiness the quantity of evaporation from 

 any surface in a given time, is an important acquisition, not 

 only in meteorology, but in agriculture and the various arts and 

 manufactures.^ — The rate of exhalation from the surface of 

 the ground is scarcely of less consequence than the fall of 

 "rain, and a knowledge of it might often direct the farmer 

 advantageously in his operations 



I may add, that from the experience I have had with 

 this instrument,* I am satisfied that it is liable to few or 

 no objections ; it affords most delicate and accurate infor- 

 mation, is portable, and is not liable to derangement. It 

 is only necessary to be very careful not to let the porous ball 

 be soiled, which would be apt to clog up its pores. It ought 

 therefore never to be touched even with thehand, and the in- 

 strument must therefore be lifted or carried by its glass 

 tube. When not used it ought to be kept in a box, with 

 its ball naked, for if wrapped in paper, part of the paper is 

 apt to adhere to it. 



Being acquainted with no method of ascertaining, with 

 the slightest degree of accuracy , the electric condition of the 

 atmosphere, I have not considered it necessary to leave any 

 column for registering it in the meteorological table. The 



* I carried one with me from Eugland through the whole of Egypt, and a 

 great part of the south of India, aud it coatiaues as perfect aad acts as weii as 

 on the iirst day I used it. 



