120 Mr. J. Y. Buchanan. On Rapid Variations of [May 31, 



vessel, in it. If T be the "range" in still air, and t the observed 

 " range," then the velocity of the wind is 



20 T t c 



v = . in feet per second, or 



3 t 



y 



v = X 4^ in miles per hour. 



t 



Converting into metrical units we have 



T-t 



v = 2'032 - m metres per second. 

 t 



If in our experiments we ascribe the whole difference in the rate of 

 cooling in the room and in the open air to motion of the air, and 

 apply Leslie's formula, we find that the air must have been passing 

 the thermometer at the rate of 0'5, 1'6, and 2'5 m. per second respec- 

 tively. On each occasion there was no perceptible horizontal motion 

 of the air, and the differences in the rates of cooling observed may, in 

 the absence of a better explanation, be held to indicate the presence 

 of ascending or descending currents of probably very local character. 



In the winter of this year I revisited the Engadin, and stayed for 

 a fortnight at St. Moritz. As the room which I occupied faced due 

 north the window of it was convenient for making observations of 

 the temperature of the air. From the 24th February to the 

 3rd March I made every morning a series of observations of the 

 temperature of the air, beginning when there was just light 

 enough to read the thermometer, and continuing till between 8 and 

 9 o'clock in the morning. At first I took the temperature every 

 minute, but finding the oscillations of temperature very great, I 

 reduced the intervals to twenty seconds, and sometimes to fifteen 

 seconds. The thermometer used was the one whose " ranges " in the 

 still air of a room and outside have been given above. As before 

 remarked, it is a sluggish instrument, yet the variations which it 

 indicated in these short intervals of time were much greater than 

 I could have anticipated. To print the observations in extenso would 

 occupy too much space, but the striking features can be easily sum- 

 marised. They are given in Table III. Excepting on the 26th 

 February, when it was snowing all the morning, the observations 

 embrace the interval of an hour or an hour and a half after sunrise. 

 The time was devoted entirely to this object, and observations were 

 made at as close dates as possible. Working alone, an interval of 

 twenty seconds is quite convenient ; shorter intervals cause hurry. The 

 time immediately following sunrise is when one would expect the tem- 

 perature of the air to rise continuously, if not regularly ; but we see 

 that so far from rising continuously and regularly the thermometer 



