KErORT OF THE KEW COMMITTEE. XXXVU 



IV. 



Kew Observatory, Richmond, July 4t,h, 1864:*. 



Mr DEAR Sre, — I have perused Mr. Ally's letter to you, in which he states 

 that the passage in his lieport to the Board of Visitors, about which you 

 wrote to him as Chairman of the Kew Committee, was intended to refer to 

 the dip-obseiTations made at the Kew Observatory, and published in the 

 publications of the Koyal Society. I have likewise perused your reply, and 

 now, in accordance with your request, I shall describe the mode of dip- 

 observation at Kew, in order that you may see that Mr. Airy's remark is 

 inapplicable to our determinations. 



But before doing so it may be well to state that the list of dip-observa- 

 tions recorded in the publication to which Mr. Airj- refers is a faithful and 

 complete catalogue of those which have been made at this Observatory. My 

 connexion with the publication referred to is therefore this : I look upon it 

 simply as an authorized and compendious catalogue of the dip-observations 

 which have been made at Kew; and regarding the method in which tlieso 

 have been discussed in the publications of the Eoyal Society as not falling 

 within the scope of my reply, I shall confine myself -to the question of mental 

 bias, and endeavour to show you that our dip-observations are quite free 

 from any such source of error. 



In the first place, the circles used at Kew are all of the same pattern ; 

 this being one which combines the united experience of several eminent 

 magneticians, and which they were several years in bringing to perfection. 

 The circles and needles are all likewise made by the same optician (Mr. Henrj^ 

 Barrow), who has devoted very great pains to the construction of these instni- 

 ments. I mention this latter circumstance, because in this observation it is 

 absolutely essential to have a needle constructed with the greatest care. 

 Before commencing the observation, the fine hard axle of the needle is 

 gently inserted into a piece of soft cork, in order that it may be thoroughly 

 cleansed, and the agate knife-edges upon which it is to rest are likewise 

 rubbed with cork The needle itself has been previously magnetized by being 

 rubbed ten times on each side from centre to pole by a pair of bar magnets. 

 After the plane of the magnetic meridian has been determined in the usual 

 way, the circle is placed in this plane, and the needle is observed in the four 

 following positions : — 



I Face of needle to face of instrument .... Face of instrument East. 

 II. „ ,, » ■■■■ „ .. West. 



III. Face of needle reversed .... „ „ West. 



IV. „ „ .. East. 



The poles of the needle are then reversed by ten strokes of the bar magnets 

 on each side, and the same set of observations is repeated, the mean of the 

 Avhole eight positions giving the dip. 



Both extremities of the needle are in each case successively viewed by 

 microscopes attached to an arm, which also carries the verniers by means of 

 which the position is read. Before making an observation, the needle _ is 

 gently raised frorii its support and lowered again by means of a lifter twice 

 or thrice, after which its position is noted. I ought likewise to remark that 

 in magnetizing the needle it is always placed in a wooden frame in such a 

 manner that the magnets are obliged to pass symmetrically over it. 



In this process it appears to me that the only possible effect a mental 



* This letter, althovigli wi-itten on July 4th, was not sent to Mr. Airy until it had been 

 approved of by the Committee at their meeting on August 26tb. 



