REPORT OF THE KEW COMMITTEE. XXXV 



ttat, in the interest of Magnetical Science, the precise value of dip-observa- 

 tions made in this Observatory should be definitely ascertained. 



Believe me, my dear Sir, 



Yours very truly, 

 To 0. B. Airy, Esq., F.R.S., (Signed) J. P. Gassiot, 



Astronomer Royal, Observatory, Greenwich. Chairman. 



II. 



Eoyal Observatory, Greenwich, S.E., 28tli June, 1864. 



Mr DEAR SiE, — I have to acknowledge the receipt of your letter of 27th 

 inst., in which you state that the attention of the Kew Committee has been 

 drawn to a paragraph in my Eeport to the Visitors of the Royal Observatory, 

 wherein I express my opinion on the inaccuracy of the small probable errors 

 which have been attributed to ordinary dipping-needles ; and in which you 

 fm-ther remai'k that the cited paragraph may be considered to refer to other 

 observations than those made at Greenwich, and therefore, on the part of 

 the Xew Observatory Committee, you inquire whether the paragraph in 

 question is intended in any measure to refer to dip-observations made at the 

 Kew Observatory, and published in the publications of the Eoyal Society ; 

 the object of the Committee being that, in the interest of Magnetical Science, 

 the precise value of dip-observations made in the Kew Observatory should 

 be definitely ascertained. 



It gives me great pleasure to enter fully upon any matter to which you may 

 invite my attention, and particularly so when the object is such as is charac- 

 terized in the last paragraph of yoiir letter. 



The inquiiies in your letter are in fact two ; — 



First. Whether the paragraph of my Report refers to other observations 

 than those made at Greenwich ? 



To this I reply that it necessaiily refers to other observations. I have 

 never succeeded in producing the agreement of results which is implied by 

 the smaUness of the probable errors, except by unfair selection among the 

 discordant primaiy elements of obsei-vation on. which the result is founded. 

 T have stated this repeatedly in my Reports to the Board of Visitors (the 

 whole series of which, I believe, are lodged in the Kew Observatory), and 

 I have in one at least particularly remarked that the discordance stiU 

 exists with the very fine instrument now in use at the Royal Observatory. 



Second. Whether the paragraph of my Report is intended in any measure 

 to apply to dip-observations made at the Kew Observatory, and published in 

 the publications of the Royal Society ? 



To this I reply that it is intended so to apply, inasmuch as the degree of 

 accuracy, to which I do not give my assent as real or weU founded, is claimed 

 for the dip-observations made at the Kew Observatory. In support of my 

 statement of that claim, I wiU refer to a pamphlet by General Sabine, which 

 I am unwilling farther to describe, but which, as I am aware, has been 

 forced on your attention and on that of the other members of the Committee 

 of Recommendations of the British Association. In it wiU be found the fol- 

 lowing sentences: — "The probable error of a single observation of the dip 

 with reliable instruments of easy procurement is known to be + V-5. It 

 has been shown to be so by a series of 282 observations made at Kew, em- . 

 ploying 12 circles and 24 needles, all of the pattern which has been in use 

 at Kew for several years past. The observations were made by seven different 

 observers : the results are published in the ' Proceedings of the Royal Society,' 

 March 1861, from entries in the Kew Observatoiy books, not a single ob- 



c2 



