216 Mr Barlow's illustration of Facts connected with 



28 of my first edition are quoted, to show that I in " all cases'" 

 so made my reference. I undoubtedly generally made my 

 reference in this way ; but still, in page 51 of the same edi- 

 tion, in the " General Summary of my first series of Experi- 

 ments,''' I have stated, that, ** instead of conceiving the imagi- 

 nary sphere to surround the ball, we may imagine a similar 

 sphere concentric with the pivot of the needle ; then it is ob- 

 vious, that the centre of the ball will have the same relative 

 position on the latter sphere, as the pivot of the compass has 

 with respect to the former ; so that the reference may be made 

 to either at pleasure." 



In fact, whether I had stated this inversion or not, it does 

 not necessarily alter one step in any investigation, nor change 

 a single letter or sign, whether the reference be made to one 

 centre or to the other. It is obviously the idea of employing 

 latitude and longitude of position by means of an imaginary 

 sphere, instead of the usual position by rectangular co-ordi- 

 nates, that is meant, where it is said that " Mr Christie, 

 adopting the views of his friend Mr Barlow, by conceiving an 

 ideal sphere to surround the compass," &c. ; and I must think 

 that common courtesy required this reference. 



Mr Christie's next remark, which applies rather to myself 

 than to the reporter, is certainly a very singular one, viz. that 

 notwithstanding I waited from December to May, that our 

 papers might appear together, I afterwards rendered that ac- 

 commodation nugatory, by permitting a report of my experi- 

 ments to appear in the Edinburgh Journal for the July fol- 

 lowing, that is, two months after the papers were read, and the 

 very day on which two distinct reports of Mr Christie's expe- 

 riments were published in two other quarterly journals ; and 

 he at the same time expresses his surprise how it should 

 be known in Edinburgh, (although I had been there just 

 before^ that this delay had taken place. The following ex- 

 planation, therefore, does, upon the whole, appear to be ne- 

 cessary . 



When the thought had occurred to me to make my experi- 

 ments on the rapid rotation of the iron ball, and that I had 

 obtained the result, (some months after Mr Christie had given 

 the first sketch of his experiments in the Phil. Trans.) I men- 



