Heat Loss by Convection from Wires in Stream of Air. 899 



The condition that the constant o£ the instrument should 

 be zero may be conventionally represented with fair approxi- 

 mation by 



B = 2 + « + 10' 

 and this is satisfied if 



* = "7, /e'=-85, J = 1-28, 



«=l-47, «' = -l-3, I = '81. 



The leading differences between the two series are that 

 when the focussing lens is positive the powers are com- 

 paratively small, and therefore larger apertures may be 

 employed ; and, on the other hand, when a negative focussing 

 lens is used the length of the telescope is reduced, the system 

 resembling a tele-photo lens. It is doubtless this feature 

 which has led to the general adoption of the negative lens. 



In, this investigation no account has been taken of the 

 errors arising through imperfect focussing of the system or 

 inexact reading. The latter introduces an error of the same 

 general type as 7, and a possible error of this type is therefore 

 always present. If care is taken in use, the errors due to 

 inaccurate focussing should not be large, and when the 

 focussing lens is positive it is possible, provided the system 

 is properly corrected for aberration, to adopt a construction 

 which ensures that the possible errors from this cause are 

 quite negligible. 



The National Phvsical Laboratory. 

 January, 1920. 



LXXXY. Ike Heat Loss by Convection from Wires in a 

 Stream of Air, and its Relation to the Mechanical Resistance. 

 By A. H. Davis, B.Sc* 



1. Introduction. 



IT has been shown | that some published results for the 

 cooling of cylinders in a stream of air agree excellently 

 with Boussinesq's formula, which may be derived from con- 

 siderations of similitude J, and which may be written in 



* Communicated bv the Author. 



+ Davis, Phil. Mag. xl. p. 692 (1920). 



t Rayleig-h, Nature, xcv. p. 66 (1915). 



