44 



PROFESSOR A. CRICHTON MITCHELL ON THE 



vento uniformiter spirante." It is unlikely that the air current was produced arti- 

 ficially ; most probably it was simply a breeze of wind which Newton believed to be 

 blowing with uniform speed. Let us assume its speed to have been about eight miles 

 per hour, and examine the results of an experiment made on the copper ball at or near 

 that speed. The following table gives the rates of cooling in a current of air whose 

 speed was 271 metres per minute (8 "09 miles per hour) : — 



Rates of cooling (degrees 

 Centigrade per minute). 



40 



Temperature Excess. 

 80 120 160 



200 



39 



8-0 



12-0 



16-2 



20-6 



From these figures it follows that the rate of cooling in a current of air whose speed 

 is 217 metres per minute is proportional to the temperature excess, up to at least 120° 

 of temperature excess, beyond which it proceeds according to some law involving terms 

 higher than those of simple proportion. 



Let us now take an experiment at a higher speed. At 1031 metres per minute the 

 rates of cooling were as shown below : — 



Rates of cooling (degrees 

 Centigrade per minute). 



40 



Temperature Excess. 

 80 120 160 



8-9 



17-8 



26-7 



35-6 



200 



44-5 



Hence the rate of cooling at the higher speed of 1031 metres per minute is pro- 

 portional to temperature excess up to at least the higher temperature excess of 200°. 

 Other experiments confirm this result, and justify the following general statement — at 

 least within the limits of 200° temperature excess, and an air-current speed of 1000 

 metres per minute : — 



When a heated body is placed in a current of air of uniform speed the rate of 

 cooling is proportional to the temperature excess, up to a temperature excess which 

 increases with increasing speed of the current of air. 



Another way — and from a historical point of view the more interesting way — of 

 stating the result is, that 



Newton's Law of Cooling is accurate under the conditions premised by him, 

 provided the speed of the current of air passing the surface of the cooling body be 

 sufficient. 



It is necessary that this result should if possible be explained. I do not, of course, 

 mean that the Law of Cooling generally is represented as above. Cooling is not the 



