On the Correction of the Compass in Iron Ships. 267 



at the centre of the ball, 

 at one fifth of the radius from the centre, 

 at two fifths „ „ n 



at three fifths „ „ „ 



at four-fifths „ „ „ 



at five fifths „ „ „ — that is, 



at a point on the circumference. 



In this figure (6), OP represents the initial excess of tempe- 

 rature of the whole ball over that of the stream of water, and 

 Q, measured along the axis of time, represents 2500 seconds 

 from the commencement of the cooling. 



The curves G G G, H H H, 1 1 1, J J J, K K K, L L L,fig. 7, 

 represent respectively the distribution of temperature from 

 the centre to the circumference of the ball at times 100, 500, 

 1000, 1500, 2000, 2700 seconds from the commencement of 

 the cooling, O T representing the initial difference between 

 the uniform temperature of the ball and the stream of water, 

 and O R, measured along the axis of distance from the centre 

 of the ball, representing the radius. 



We have to thank three of our students, Messrs. Asano, 

 Fujioka, and Nakano, for assistance rendered in the experi- 

 ments, and three others, Messrs. Nakahara, Nobechi, and 

 Oshima, for aid given us in the calculation of the Table of the 



values of -. for different values of a. 



tana 



January 1878. 



XXXVII. On the Correction of the Compass in Iron Ships 

 without use of a Fixed Mark. By Sir G. B. Airy, K. C.B., 

 Astronomer Royal *. 



[Plate XI.] 



I AM indebted to Sir William Thomson for the suggestion 

 that the Compass in an Iron Ship might be corrected for 

 the effects of the Permanent or Polar Magnetism of the ship 

 without use of a fixed mark. On considering the subject, the 

 process here described quickly suggested itself. It is based 

 upon the following assumptions : — 



That the effect of the Transient Induced Magnetism may be 

 neglected. 



That by means of an auxiliary compass the ship's head 

 may be kept steady on one bearing for a few minutes. 



That the magnitude of the actual directive force may be 

 ascertained, in terms of any arbitrary scale (the most con- 

 * Communicated by the Author. 



