REPORT or THE KEW COMMITTEE. xliii 



of being applied in conjunction with the horizontal force to several important 

 problems in which the theoretical bearings of the variations of the dip and 

 total force are concerned, which will be shown as soon as the reductions, 

 already far advanced, are completed; meanwhile instruments of the same 

 pattern have been ordered by the directors of several foreign observatories, 

 who have themselves personally examined the Kew instruments and the 

 records of their performance, and have expressed their intention of working 

 in concert with Kew. 



The displacements and dislocations which have occasioned Mr. Aiiy so 

 much trouble for several years past in the Greenwich vertical-force instru- 

 ments are obviously due to a cause or causes very different from that which 

 has been noticed above. From his own description of them, we learn that 

 the results in one sheet cannot be compared with those in another, and that 

 in 1859 the vertical-force magnet exhibited for the daily magnetic curve a 

 form approaching much more nearly to a straight line than it had usually 

 given. The imperfection of such an instrument is sufficiently manifest, 

 and it would not be difficult, perhaps, to assign its probable cause or causes; 

 but as it is no longer designed to be used by Mr. Airy himself, I submit 

 that it would be inexpedient to employ the time of the observatory in in- 

 vestigating how much the defect of an instrument which is given up by its 

 employer may bo due to one cause and how much to another. The Kew 

 instrument has no such defect ; in other words, it is, to use Mr. Airy's 

 expression, " incompetent to exhibit the displacements " (or dislocations) 

 which take place in the Greenwich instrument. 



Again, in order to investigate these dislocations experimentally, it would 

 be necessary that the Committee should dismount our present insti-ument and 

 mount one similar to that which Mr. Airy has discarded, if not that very 

 mag-net itself, and Mr. Airy in his request intimates that some such change 

 would be necessary. To dismount an instrument so usefully employed as that 

 at Kew, and with the performance of which for the purposes for which it was 

 devised we have reason to be fully satisfied, for the chance of^ constructing 

 one of a different form, which might probably not give us equal satisfaction, 

 would seem to be a species of treason to the branch of science which we 

 are endeavoimng to advance, as well as to ourselves, and to those who have 

 provided themselves with similar instruments to work in concert with us. 



3. Mr. Airy's third request is that we should make experiments in order 

 to determine if there be any difference in the temperature correction as de- 

 rived when the magnet employed is placed in hot and cold air instead of in 

 water, as is usually the case. 



Let me first of all direct your attention to the principle on which the Kew 

 Committee have proceeded for several years past in reference to the subject 

 of temperature corrections. This principle has been to avoid, as far as 

 jjossible, the occasion for such corrections, and the Committee will be glad to 

 learn that Mr. Airy has latterly expressed his intention of adoptiog the same 

 principle. At the Kew Obseiwatory the variation of temperatm-e to which 

 the magnetographs are exposed is ouly half a degree Fahr. in twenty-four 

 hours. In like manner, in the instniment for absolute determinations, by 

 making the deflections and vibrations sufficiently near to one another in 

 point of time, the correction for temperatiu-e is reduced to a minimum. 



But in former days a number of experiments were made on the temperature 

 correction, some with the purpose of proving that magnetic changes are not 

 caused by the varying temperature of the aii-, and others which exactly cor- 

 respond to the point referred to by Mr. Airy, and these lead to the belief that 



