REPORT OF THE KEW COMMITTEE. xli 



I -will take these rcqiiests in succession, 



1. His first relates to dip experiments and observations. Abont tAVcnty- 

 five years ago, a few magneticians, including General Sabine and the late ISir 

 J ; C. Ross, M'ho were zealous for the advance of magnetical science, set them- 

 selves to vrork to improve the dip-circle. In this problem they had the ad- 

 vantage of the cooperation of the late Mr. Robinson, an excellent mecha- 

 nician, who had also the subject very much at heart, and whose attention 

 was especially directed to the a.vle of the needle with remarkable success. 

 On his premature death, his process was continued by Mr. H. Barrow. Other 

 improvements were afterwards made, and the Kew Observatory having in 

 the mean time been estabHshed, that institution was not slow to recognize 

 the practical excellence of this circle, and the Committee felt themselves 

 able to recommend its general adoption. In order to justify their preference, 

 they instituted a series of monthly obsei-vations, the result of which, in their 

 opinion, as weU as in that of very many scientific men, has been to demon- 

 strate the practical goodness of this instrument. Not fewer than forty-two 

 of these instruments have been made by Mr. Barrow, and these are, for the 

 most part, in use in different parts of the globe. Many directors of foreign 

 observatories who were previously acquainted with other dip-circles, suspect- 

 ing these to be inferior to that at Kew, have repaired to our observatory for 

 the purpose of convincing themselves by their own experience that the per- 

 formance of the Kew circle was not exaggerated. I believe that, Avithout 

 exception, they have been satisfied with our results ; but I need not dwell 

 on this topic, as I have akeady in a previous letter endeavoured to show that 

 our observations arc quite trustworthy. 



It was the wish of General Sabine, who had taken such an active part in 

 dip-observations, as well as in the construction of the new circle, to exhibit in 

 a scientific manner the probable error of a complete observation of the dip with 

 any Kew instrument ; and for this purpose he requested me to furnish him 

 with a complete Hst of the results obtained at Kew since 1857, omitting none. 



These observations were printed in the publications of the Royal Society, 

 and I may be admitted to express my belief that, in the method of reduc- 

 tion employed, the observations were coinbined in the manner most approved 

 by physicists. I may likewise mention that the probable error therein ob- 

 tained, small as it is, must not be regarded as wholly due to instrumental 

 inaccuracy, but in part at least to the occurrence of disturbances during 

 some of the observations, a source of error which cannot be avoided. If 

 Mr. Airy will refer to the results of the Kew observations in the Philosophical 

 Transactions for 1863, art. 12, he wiU see an example of the advantage of 

 employing an inclinometer with the small probable error of that of Kew, in 

 problems of much theoretical importance. 



It would thus appear that the Kew Committee have already obtained an 

 almost perfect dip-ciixle, so that it is not easy to conceive what advantage 

 is to be derived from the experiments proposed by Mr. Airy, especially since, 

 in order to obtain the result which he desires, he has only to become per- 

 sonally acquainted with the working of our instrument, as has been done by 

 those scientific men who have already visited Kew for this pm-pose. 



2. Mr. Airy states, — " I have been troubled for many years with small 

 displacements in the trace of the vertical-force photograph. I should be 

 glad to have these investigated at the Kew Observatory ; but it wiU be neces- 

 sary for this purpose to modify the adjustments of the vertical-force instru- 

 ment at Kew, which at present is incompetent to exhibit such displacements, 

 and masks aU that may ever have occurred." 



