REPORT OF THE KEW COMMITTEE. XXXV 



" Trusting that you will take the matter into consideration, and excuse the 

 liberty I have taken in addressing you, 



" I am, Sir, 

 " Your obedient Servant, 

 (Signed) " W. Scott, 



" Astronomer for N. S. Wales." 

 " Major- General Sabine." 



" 13 Ashley Place, London, May 8, 1860. 



" Sir, — I lose no time in replying to your letter of March 2, received this 

 day. The self-recording magnetical instruments at Kew have been in action 

 nearly two and a half years — a sufficient time to test their merits or defects. 

 I have myself completed the analysis and reduction of the first two years (1858 

 and 1859) of the Observations of the Declinometer, and can therefore speak, of 

 my own knowledge of their performance, as far as that element is concerned. 

 The Photographic Traces, recording both the zero line and the actual move- 

 ments of the magnet, can be measured with tolerable confidence to the third 

 place of decimals of an inch, the inch in the Kew instrument being equiva- 

 lent to 22 minutes of arc. The reading is consequently made to the 1000th 

 part of 22 minutes of declination. The record is of course continuous ; but, 

 for the purpose of computing the results, hourly readings have been tabulated. 

 In the first year the trace failed in 107 out of 8760 hours, chiefly from 

 failure in the supply of gas, which is brought by pipes from Richmond, a 

 considerable distance off. This inconvenience has been remedied by the 

 construction at the Observatory itself, at a small expense, of a water regu- 

 lator, through which the supply from Richmond passes, and there is now no 

 reason why the trace should ever fail. I have now in course of analysis and 

 reduction the same years of the observations of the horizontal and vertical 

 force magnetographs, and have no reason hitherto to believe that the record 

 of those two elements will be inferior to that of the declination. The three 

 instruments, with the clock which keeps the registering papers in revolution, 

 together with reading telescopes placed for eye observation, either to accom- 

 pany or to be independent of self-registry, occupy an interior space of about 

 16 feet by 12, including a passage round for the observer. The cost of such 

 a set of instruments, complete in every respect, is £250 ; and four months 

 must be allowed for making them from the date of the order, as well as an 

 additional month for their careful verification at Kew (should that be de- 

 sired), where a detached building has been erected for this particular pur- 

 pose, in which they may be kept in work in comparison with the Kew instru- 

 ments. A detailed description of these instruments is now in the press, and 

 will be published in June in the volume of Reports of the British Association. 

 The results of the first two years of the Declinometer observations, showing 

 what are deemed at present to be the most useful modes of eliciting the re- 

 sults, will be printed in the • Proceedings of the Royal Society' in the present 

 summer, and the first two years of the horizontal and vertical force magneto- 

 graphs in the same publication later in the year. A small adjoining room is 

 requisite, opening if possible into the instrument-room, which should contain 

 suitable troughs for the preparation of the paper to receive the traces, and 

 to fix them. It is important to diminish as much as possible the changes of 

 temperature in the Observatory itself, exclusive of the effect of the instrument 

 cases, which have adaptations for that purpose. So far in regard to differential 

 instruments. For absolute determinations and secular changes a small de- 

 tached house is required, say 12 feet by 8, in which equality of temperature 

 need not be regarded, but which must be at a sufficient distance from other 



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