158 



Dr. C. Chree. 



Inclination . 



Kew Observatory — 



Date. Time. Observatory. Moureaux. Moureaux. 



July 27 11.15—11.41 a.m. 67° 20*2' 67° 18-9' + 1'3' 



28 11.21—11.43 ., 20'3 18*7 +1*6 



29 8.55— 9.18 „ 19"8 173 + 2"5 

 29 9.21— 9.43 „ 20*0 17'6 4-2-4 

 29 9.47—10.9 „ 20*0 18*0 + 2'0 



Mean + 2-0' 



In judging of the results several tilings merit consideration =- 

 Neither inclinometer read to less than 1', decimals arising from 

 arithmetical operations. The Kew unifilar magnetometer reads to 

 10", but with. M. Moureaux's much, smaller instrument readings 

 could not be taken to less than 30". The great skill of the two- 

 observers is beyond question, and the mean of several results 

 obtained without mental bias may possess an accuracy greater than 

 that which any individual reading can lay claim to. Still, personally, 

 I should be very sorry to claim accuracy of the order of 1 in the 

 last significant figure of the mean differences. 



In the use of the results, one should remember the possibility, or 

 rather probability, of the occurrence of change in magnetic instru- 

 ments. This is a vicissitude to- which travelling instruments are 

 probably most exposed, but even in an observatory standard it is 

 certainly not impossible,. The constant " P," appearing in the 

 factor 1 — Pr~ 3 , which allows for the departure of the horizontal 

 force magnet from the ideal infinitely short magnet, appears to be 

 to some extent variable, at least in the Kew instrument. This par- 

 ticular variation does not necessarily affect the calculated horizontal 

 force, because the proper value of " P " for a special set of obser- 

 vations can be calculated from the experiments themselves. This 

 precaution was, in reality, actually adopted in the present case. 

 Still the fact that a change does take place, which, if undetected, 

 would appreciably influence the results, shows that assumptions of 

 absolute constancy are at present acts of faith, not reason. Until 

 there exists a regular system of intercomparison of the instruments 

 at different observatories, our information on this head is likely to be 

 limited. 



Pavlovsk (St. Petersburg) and Uccle (Brussels) — lie lias made use of new values 

 for the constants of the French instruments, which it is intended to apply con- 

 sistently after Jan. 1, 1898. 



This change, entailing a reduction of O00067 C.G-.S. unit in the corrected 

 readings of the French standard, was postponed until the French survey should be 

 completed, though the necessity for it has been recognised for some years. Par 

 ticnlars will be found in a paper by M. Moureaux in the ' Annales du Bureau 

 Central Meteorologi^ue de France,' vol. 1, 1896, now in the press.] 



