THEORY CONFIRMED BY PHENOMENA. 75 



temperature of the air is above the dew point c.t the mo- 

 ment of their formation. 



If gentlemen have no means of taking the dew point 

 directly, the following method will be found equally correct 

 in ascertaining the height of the base of these particular 

 clouds, at any time of the day, for the height varies every 

 hour. Swing a thermometer (Fahr.) rapidly in the air to 

 avoid the effect of radiation, note its temperature, then cover 

 its bulb with a wet rag and swing it as before until it sinks 

 as low as evaporation can make it, then divide one hun- 

 dred and three times the difference of these temperatures 

 by the wet bulb temperature, the quotient will be the height 

 of the base of the clouds in question, in hundred yards. For 

 example, suppose the dry bulb is 56|, and the wet one 51|, 

 then the base of the clouds will be one thousand yards high. 

 This height is calculated on the supposition that air cools at 

 5 Fahr. in ascending to a height where the barometer would 

 be one inch lower than at the surface of the earth, and 5 

 more for every additional inch. If this latter law is not 

 strictly correct, the height of the base of the cloud in ques- 

 tion will vary accordingly, and the law itself may be accu- 

 rately investigated by this method, for the precise degree 

 of refrigeration necessary to condense vapor at a particu- 

 lar dew point is known, after making an allowance for the 

 expansion of the vapor itself and the fall of the dew point 

 on this account. As the discovery of a method to ascertain 

 by the thermometer the height of a particular kind of cloud, 

 easily distinguishable from all others, is a matter highly 

 curious in itself, independent of its connexion with the 

 theory here advocated, it will no doubt receive that imme- 

 diate attention which it deserves. 



97. Since writing the above a kite was sent np into the 

 base of a cloud, and its height ascertained by the sextant, 

 and compared with the height calculated from the dew 

 point, allowing one hundred yards for every degree the 



