REPonT or THE kew committee. xlv 



4. The vibrations of the magnet arc not well shown. The largest are those 

 of June 9'^ 2" 10'" (some doubt about this), June 20'' 14'^ 1 7'", and July 30" 0'' 8"^; 

 all these are really small, yet they are exhibited very feebly. It seems pro- 

 bable that a larger vibration would leave no visible trace. 



5. I conclude from this that very violent and rapid changes of magnetism 

 could not be shown. 



6. In this respect the process used at Greenwich (fully detailed in the 

 " Magnetical and Meteorological Eesults," 1862), which appears to be more 

 sensitive to rax)id movements, seems preferable to that used at Kew. I do 

 not propose to make any change for our magnetic instruments ; although for 

 our exposed thermometers, in which the changes are not sudden, is yet under 

 consideration whether a process hkc that of Kew shoidd be introduced. 



7. I find upon close inspection that the Kew curves are not free from dis- 

 locations ; these are, however, smaller than those of the Greenwich curves. 

 A few of them have caught tlie attention of the Kew observer, and are in- 

 dicated by dots of red ink. Among these, I think, are June 17" 23'' 50", 

 and June 19" 22'' 40'". But there are many others (all small), as Jime 

 23" 23" 40"\ June 29" 23" 0'", July 1" 21" 30™, July 3" 22" 50'", 23" 30'", 

 23'' 35'", &e. etc. ; July 13" 5" 0'" (which I note as occurring at a different hour 

 of the day), &c. These are imimportant as affecting the readings of the 

 curves, but not unimportant as affecting the possible explanation. 



8. The comparison of the readings at Greenwich and at Kew, on days when 

 tlie dislocations at Greenwich are sensible, entirely supports the view which 

 I have entertained for many years, that the dislocations are transient phe- 

 nomena, in no wise affecting the zero-measurement, and whose effects can by 

 judicious attention be entirely remedied. 



I am, my dear Sir, 



Yours very truly, 

 John P. Gassiot, Esq., (Signed) G. B. Airy. 



Chairman of the Keiu Committee. 



IX. 



Eoyal Observatory, Greenwich, S.E., 18G4, November 15th. 

 My dear Sir, — You are aware, perhaps, that Mr. Glaisher has visited 

 the Kew Observatory, and that Mr. B. Stewart and Mr. "Whipple have 

 visited this Observatory, and that two of the Kew Dip iustniments have 

 been transported backwards and forwards ; and that observations have been 

 made \nih. them by all the gentlemen Avhom I have mentioned, at Green- 

 wich and at Kew ; and that during these operations I have myself carefully 

 examined the principal parts of the instruments, though I have not made 

 any complete observations with them. The result of these operations is as 

 follows : — 



1. As far as depends on the mechanical construction, of the instruments 

 including the needles, the workmanship of the instruments (I am not now 

 speaking of the extent of applicability but of the workmanship of the important 

 parts) is very good, of the same class as ours ; I think ours better in some 

 respects, but they may be considered as the same class. 



2. As regards the results of observations, those made with the Kew instru- 

 ments are consistent to a degree which I never saw before. And in the 

 experiment which, as made with our needles, has perplexed me most, namely, 

 that of rotating the instrument in azimuth Avithout touching or lifting the 

 needles, and remarking the change in their indications, the Kew needles 

 appear to be nearly or entirely free from such change. 



