of the Approach and Recession of Bodies. 71 



motions to the effects of evaporation and condensation. He 

 found that when he inclosed a little balance within a water 

 vacuum of from i to f inch pressure and approached a flame, 

 the nearest pith-ball was driven away ; but that when a 

 piece of ice was brought near the ball swung towards the 

 ice. Professor Reynolds finds this behaviour to be directly 

 deducible from the Kinetic theory of gases. A material 

 surface in the act of throwing off vapour is itself repelled by 

 the reaction, whilst a similar surface condensing vapour has 

 the pressure on that surface diminished, and is consequently 

 drawn forwards. 



The author then proceeded to describe a few of the 

 experiments he was led to make whilst examining these 

 phenomena, in the following detail : — 



" In order to examine these motions in air at the ordinary 

 pressure, a large glass shade, about two feet high and eleven 

 inches in diameter, was employed. This was inverted and 

 the open end covered with a glass plate. By the aid of a 

 fibre of raw silk, there was suspended horizontally within it 

 a balance consisting of a glass stem carrying cylinders of 

 pith at the ends, and hanging about 13 J inches from the 

 glass plate. 



" Experiment 1. — When a flame or other source of heat is 

 brought near this glass shade, the nearest pith approaches 

 the source of heat. It swings steadily towards the flame, at 

 a more or less accelerating speed, until it nears the flame, 

 when it begins to slow. It usually passes some degrees 

 beyond this point, due to momentum as it might be, but 

 returns and comes to rest within some degrees of the flam a 

 The behaviour, however, varies somewhat, the balance in 

 some cases slows and stops before the pith ever reaches the 

 flame, sometimes it keeps slowly vibrating from side to side 

 of that position, but when it comes to rest it is in almost 

 every instance from 5° to 10° on one side of the flame. 



" This suggests the action of two or more opposing forces 

 simultaneously at work upon the pith, the balance taking up 

 a position determined by the resultant of these forces. 



" Experiment 2. —Another observation that supports this 

 view of the case is this : — 'When a basin of water 

 is placed within the glass shade, the atmosphere of the 

 interior soon becomes saturated with water, and whenever 

 the temperature sinks below the dew-point, the inner surface 

 of the shade becomes bedewed with moisture. If now the 

 flame be approached when the shade is in this condition, the 



