xxxviii REPORT — 1864. 



bias can be imagined to have is to induce the observer to continue lifting 

 the needle before reading, until it has come into what he considers the 

 proper position ; but even this is totally precluded by the method of observa- 

 tion, for the vernier is not read, and the observer does not know the position 

 of his needle until it is at rest and the lifting process at an end. Besides, 

 if the obseiwer did know the position of his needle it would avail him little ; 

 for while the mean of the eigJit positions is nearly the same for different 

 instruments, yet the reading of any one position of the needle may be, and 

 usually is, very different from the true or hnally deduced dip. 



Prom all this it will be seen how little scope there is in the dip-observa- 

 tions for the operation of mental bias ; but the observers who are supposed 

 to have worked our instruments with an unconscious predetermination to 

 produce certain results must have had still more formidable difficulties than 

 even these to contend with. Por, in order that mental bias should have 

 operated in the case under discussion, the preconceived idea of uniformity 

 with which the observer approached the instrument must have varied in such 

 a measure from season to season and from year to year as to produce in the 

 results obtained an annual variation, as well as a secular change, and these 

 of such a nature as to conform with the results of other observatories. Mr. 

 Airy must acknowledge that the uniformity to which he alludes, and the wish 

 for which he supposes has created a mental bias, is that which remains after 

 the annual and secular variations have been allowed for. 



Next, with regard to obsei-vers ; we have frequently at Kew gentlemen 

 connected with foreign obsei-vatories, who come to receive a magnetical 

 equipment. Their desire is to obtain the best possible instruments, but at 

 the same time they view those presented to them with a very critical eye. 

 One of these was Dr. Bergsma, who spent nearly a month in thoroughly ex- 

 amining the dip-circle and in suggesting refinements, but who went away 

 convinced of its accuracy. Seuhor da Soiiza of Coimbra, and Senhor Capello 

 of Lisbon, have likewise made dip-obsei-vations at Kew, and with the same 

 object, namely, to satisfy themselves by their own practical experience as to 

 the best dip-circle with which to furnish their respective observatories. 



I shall only allude to one observer more, who, though he only made a 

 single observation, has frequently expressed his wish to make a series, but 

 has hitherto been prevented by his numerous engagements. I speak of Mr. 

 Glaisher, of Greenwich Observatorj-, who, on 21st October last, obtained 

 with Circle No. 40 a dip of 63° 12' -2, whHe with Circle No. 33 Mr. Chambers 

 on 19th and 20th October obtained 68° 12'-3. 



I have thus endeavoured to show that in the Kew dip-observations there 

 is absolutely no opportunity for mental bias to act, and that even if there 

 were, many of our observers are not likely to have been the subjects of such 

 an influence. 



In thus fidfilliug your request, it is -^dthin my province to notice the 

 second part of Mr. Airy's letter only in as far as this is connected with the 

 subject of discussion. You will, therefore, perhaps permit me to refer you 

 to the following paragraph of his letter, which 1 shall now quote : — ■' I 

 have therefore no novelty to claim, except the suggestion (made by me some 

 years ago) of instability in the position of the magnetic axis, and the con- 

 stniction (within little more than a year) of an instrument whose results 

 appear to support that suggestion. 1 should be much gratified if the powers 

 of the Kew Observatory could be devoted to the examination of this and 

 analogous instrumental difficulties. These experimental inquiries are not weU 

 suited to the system of the estabhshment over which I preside. And, speak- 



