248 Professors W. E. Ayrton and J. Perry on 



When a is known, we can find K ; for 



K* 2 



mss Q?' 



or 



Also 



K 



Er 

 K 



(8) 



= 1 



tana 



so that E may be fonnd. After the following illustrations of 

 the method described above, we shall consider the possible 

 errors involved in calculating K and E in this way. 



Y. As a first example, we shall take the time-curve of fall of 

 temperature of the centre of a stone ball of 5^ centims. radius, 

 which we obtained when the initial temperature of the whole 

 ball was 70° C, and the external temperature kept at 21°*3 C. 

 during cooling. For reasons given further on, the first point 

 we employ in the curve is when t equals 616 seconds from the 

 commencement of cooling. The values of v, m, and logN 

 corresponding with this and subsequent values of t are given 

 in the following Table : — 



t. 



v. 



m. 



logN. 







48-7 







616 



25-72 



000178 



1-93656 



691 



22-45 



184 



1 94149 



766 



19-4 



206 



1-94215 



841 



16-62 



196 



1-94057 



916 



13-32 



209 



1-93867 



991 



123 



195 



1-93649 



1066 



10-65 



188 



1-93795 



1141 



9-2 



203 



1-93847 



1216 



7-9 



2t>3 



193643 



1291 



677 



207 



1-93371 







Mean m 



= 0-00197, 







Mean log N 



= 1-93825, 







Mean N 



= 86-746. 



In this 



case 











sm«-«cos« 



N 





a— sin a cos a 



~2v 









86-746 









97-4 









=0-891; 



