8o Chemical and Physical Notes 



air outside is not motionless, while in a room of constant 

 temperature it is practically so. It is evident that if the 

 difference is caused by the motion of the air, then that 

 difference must also be a measure of the motion. As has 

 been pointed out above, this was perceived by Leslie, and he 

 gives formulae 1 for calculating the velocity of the wind from 

 the reduction of the term of cooling of a tin vessel holding 

 about half a litre of water. 



If R be the term of cooling in still air, or the fundamental 

 term of cooling, and r be the occasional term of cooling when 

 the air is moving with any velocity v, then Leslie gives the 

 following expressions for this velocity : 



in feet per second ; 



. R-r . ., 



or, v = 4 - in miles per hour. 



Converting into metrical units, we have 



R-r . 

 v = 2 '03 2 - in metres per second. 



It is right to observe that R in Leslie's equations is the 

 term of cooling of his flask of water when suspended " out of 

 doors, on a calm evening" 



Difference between a Calm Indoors and a Calm Out-of- 

 Doors. The preceding equations give the diminution of the 

 term of cooling produced by sensible wind as compared with 

 a calm, both being out-of-doors. It does not appear that 

 Leslie distinguished between a calm indoors and a calm 

 out-of-doors. 



Returning to Table IX at page 74, we find the term of 

 cooling of the thermometer to be in round numbers 65 seconds 

 in the still air of a room. A number of observations were 

 made with the same instrument in the open air in very calm 

 fine weather. The method of the " half-fall " as exemplified 

 in Table XI was used, the excesses of temperature used 

 being 12, 10, 8 and 6 C. The experiments were made in 



1 Loc. tit., p. 783. 



