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IV. — On the Convection of Heat by Air Currents. By Prof. A. Crichton Mitchell. 



(With a Plate.) 



(Read March 6, 1899, and December 18, 1899.) 



1. The present paper deals with a series of experiments made in the Physical 

 Laboratory, Edinburgh University, from January to October 1899, with the object of 

 determining the convective loss of heat from a cooling body owing to the action of 

 currents of air. 



2. As some of the results obtained have a bearing on the history of the laws of 

 cooling, it is necessary to refer to at least one of the investigations which have been 

 made on the subject. The first to give any definite statement regarding the law of 

 cooling was Newton. In a paper # communicated to the Koyal Society of London, his 

 experiments on cooling are detailed, and his conclusion stated as follows : — 



" Nam color quern ferrum calefactum corporibus frigidis sibi contiguis dato tem- 

 pore communicat, hoc est Color, quern ferrum dato tempore amittit, est ut Color totus 

 /em, 



Since this appeared in 1701 it has been known as Newton's Law of Cooling, and has 

 generally been reproduced in some such form as " The rate of cooling of a body at any 

 temperature is proportional to the difference between that temperature and the tempera- 

 ture of the surroundings of the body." Or more shortly, "Rate of cooling is propor- 

 tional to temperature excess." 



But at the end of his paper Newton makes the following important statement : — 



" Locavi autem ferrum non in cere tranquillo, sed in vento uniformiter spirante, 

 ut cer a ferro calefoctus semper obriperetur a vento, et cer frigidus in locum ejus 

 uniformi cum motu succederet. . . . Sic enim ceris portes cequoles cequalibus tem- 

 poribus calefoctus sunt, et calorem conceperunt calori ferri proportionalem ." 



Strangely enough, nearly all subsequent references to Newton's Law of Cooling omit 

 any mention of its most important qualification, viz., that the cooling body is placed in 

 a current of air moving with uniform speed. The only clear exception is Fourier's 

 remarks on Newton's Law in § xxxi. of his paper,t " Questions sur la theorie physique 

 de la Chaleur rayonnante," where he points out that the cooling of bodies in still air, or 

 rather in air which has no other movement than that resulting from change in density, 



* "Scala graduum caloris et frigoris," Phil. Trans., April 1701, vol. xxii. p. 824. Also in Newton's Works, 

 Horsley's edition, 1782, vol. iv. 



t Ann. de Cli. et de Physique, 1817, vi. pp. 259-303. 



VOL. XL. PART I. (NO. 4). F 



