1894.] Atmospheric Temperature, especially during Fohn. 119 



above, whereby the temperature of the air is inferred from the velocity 

 with which the thermometer rises or falls when immersed in it, 

 either at rest or moving with a known speed, is in itself quite satis- 

 factory. The difficulty in applying it is to ascertain the rate of 

 motion of the air, because, other conditions being the same, the 

 thermometer changes its temperature in proportion to the velocity of 

 the air passing it. When the air has a horizontal motion it is called 

 wind, and there are many instruments for its measurement ; but 

 there is probably nearly as much vertical as horizontal motion in 

 the atmosphere, but it is seldom observed and not easily measured. 

 In fact, a very good way of detecting these movements in air which, 

 to the senses, appears to be motionless, is to observe the rate of 

 cooling or heating of a thermometer in it. A thermometer similar to 

 the one used in these investigations was carefully tested as to its rate 

 of cooling, in connexion with a series of observations made in the 

 winter. 



Its rate of cooling was repeatedly determined in a room of constant 

 temperature, and in the open air, when it was, to all appearance, 

 motionless. Sometimes the rate of cooling in the open air was very 

 nearly the same as in the room, but at other times it was much 

 greater. It was never less. In four experiments, taking the same 

 excess of temperature above the air, namely, 5'5 C., the temperature 

 of the thermometer fell by half that amount, 2'75 C., in the room in 

 125 sees., and in the open air, which was apparently still, in 100, 70, 

 and in 55 sees. The volume of the bulb of this thermometer, which 

 was cylindrical, was 0'92 c.c. ; it was rather sluggish. 



Applying Leslie's rule for finding the " range " of the thermometer 

 from the time it takes to cool to half the extent of the difference 

 between its initial temperature and that of the air, we multiply it by 

 101/70. Leslie* defines the " range " of a thermometer or other 

 body cooling to be the reciprocal of the fraction of the whole initial 

 difference of temperature between the thermometer and the air, by 

 which it cools in the first interval of time ; or it is the time in which 

 the thermometer would fall to the temperature of the' medium, if, in 

 each successive interval of time, its temperature had fallen by the 

 same amount as in the first interval of time. The " ranges " of our 

 thermometer cooling in the above conditions are found to be 180, 143, 

 100, and 80 sees, respectively. 



Having recognised that, in the conditions under which he experi- 

 mented, the refrigerant power of a stream of air is exactly propor- 

 tional to its velocity, he givest a formula for finding the velocity of 

 the wind from the rate of cooling of a thermometer, or other similar 



* ' An Experimental Inquiry into the Nature and Propagation of Heat,' by John 

 Leslie, Edinburgh, 1804, p. 264. 

 t Page 283. 



