Xliv REPORT 18G4'. 



temperature corrections determined by hot and cold water experiments arc 

 almost identical with those determined by hot and cold air. 



I find that at Toronto the temperature change of the vertical-force magnet 

 found by comparing together days of different natural temperature was 

 •00011 for 1° Fahr., while the same dctei-mined by hot and cold water 

 experiments was -00009. At Makerstoun, also, the temperature correction 

 of the balance magnet, as determined by hot and cold days, was -000071), 

 while that determined by hot and cold water experiments was -000073. 



These agreements are very near, and the first had induced General f?abinc 

 to remark that the hot and cold water method was sufficiently correct; while 

 the same conclusion was also arrived at by Mr. Broun of Makerstoun, and, 

 as far as I am aware, has been generally received. 



It is impossible for me, after such evidence that both methods give very 

 nearly the same corrections, to doubt that Mr. Airy's vcrij great difference 

 must have been occasioned by error of experiment. 



As a principle, Mr. Aiiy will, I think, allow that in such an experiment 

 it is better to have the hot and cold air filling a whole room than filling 

 only a copper box ; while at the same time it may be extremely difficult to 

 indicate the precise source of error in his arrangement. I do not think 

 that the Kew Committee are called upon to undertake this task, especially 

 since (as has been shown) the comparison of corrections derived from heated 

 air and heated water Ims already received due attention, the result of which 

 has been to set that matter at rest in the minds of other magneticians ; and 

 also since the temperature corrections which will be hereafter required at 

 Greenwich will not be of such magnitude as heretofore, and therefore are not 

 likely to occasion Mi'. Airy the same distressing doubts as those spoken of 

 by him. 



I remain, my dear Sir, 

 J. P. Gctssiot, Esq., Yours very truly, 



Chairman of the Keiu Committee. (Signed) | B. Stewart. 



The two following letters, although of later date than the Kew Report, have 

 been attached to this correspondence by order of the Council. 



VIII. 

 Eoyal Observatory, Greenwich, S.E., 1864, October 19th. 



Mt dear Sir, — I have to thank you for your kindness in transmitting for 

 my inspection the Kew Vertical-Force Photograms for the months of June, 

 July, and August 1863. They shall be returned by hand at an early oppor- 

 tunity. I have examined them with much interest, and take leave to com- 

 municate to you the following remarks on them. 



1. The curves are traced more strongly than those which I had previously 

 seen. I think this change a great improvement. 



2. The sheets are very neat, uniform, and distinct — more uniform than the 

 Greenwich sheets have been to the end of 1^03, but, I think, not more 

 uniform than the Greenwich sheets are now. This change in the Greenwich 

 sheets, I beheve, is to be attributed entirely to our gain of nearly uniform 

 temperature, every part of our chemical process being the same as formerly. 



3. The small perturbations are recorded with great delicacy — more clearly 

 than in the former Greenwich sheets (though I believe that nearly all can 

 be traced in our curves), but not more clearly than in our new sheets. I 

 make the latter statement from examination of the general character of both, 

 as I have not been able to compare corresponding sheets. 



