ON COMPAEING AND REDUCING MAGNETIC OBSERVATIONS. 321 



It was also resolved that it is desirable to determine if the luni-solar 

 variation be dependent on the state of the sun's surface. 



Regarding this resolution, the Secretary has quite recently received 

 the following communication from Mr. C. Chambers : ' We are extending 

 the general investigation, as our limited computing force admits, to other 

 quarters, so that any variation with the sun-spot pei-iod will make itself 

 apparent in due course, and I will give you early notice of the fact when 

 an J' definite evidence becomes available.' 



The last of these meetings took place on June 30, 1887. In this 

 meeting it was I'esolved ' that in the opinion of this Committee the time has 

 now arrived when steps should be taken to obtain with as little labour as 

 possible sufficiently accurate values of the simultaneous solar-diurnal 

 variations of the magnetic elements at various stations throughout the 

 globe.' 



It is hoped that the directors of the various observatories in 

 which self-recording magnetographs are in action will join in this move- 

 ment, and in oi'der to leave them sufficient time for preparation it is 

 proposed that a commencement be made on January 1, 1889. The 

 Committee propose to confine their simultaneous comparison to certain 

 selected days for which there are reasonably smooth registers at 

 Greenwich or Kew. It is believed that these days will, in all probability, 

 be of a similar character for the other stations ; but in case there is slight 

 disturbance at any station for any of the selected days this may be got 

 rid of by the method pursued at Greenwich, where the practice has been 

 to draw a pencil curve smoothing down the irregularities of the trace. 

 Photographic records, or hourly measurements of these curves, smoothed 

 when necessary, are desired by the Committee. 



For this purpose it will be necessary to know the scale-coefficient for 

 the three magnetographs, as well as the temperature-coefficient for the 

 horizontal and vertical force magnetogi'aphs. It will likewise be neces- 

 sary to have a sufficiently accurate record of the variation of temperature 

 for each of the selected days at the self-recording chamber of each of the 

 stations. The Committee would make the following suggestions as to 

 the method of obtaining the scale and the temperature coefficients. 



(1.) Scale-coefficients. 



There can be no doubt about the scale-coefficient for the declination 

 magnetograph. 



The scale-coefficients for the two force magnetographs are determined 

 by the method of deflections, for which suitable apparatus is provided for 

 each observatoiy, and observations of deflection are made at two or more 

 different distances of the deflectors. There is an absolute necessity for using 

 two or more different distances in determining the scale- coefficient of the 

 vertical force instrument, for we have here to prove that the knife-edge 

 is sufficiently good ; and the best way of doing this is to be sure that the 

 scale- coefficient is the same both for small and for large departures as 

 determined by means of deflections at two or more diflerent distances 

 of the deflectors. 



It is desirable that the measurements of the horizontal and vertical 

 forces should be expressed in terms of absolute value in C.G.S. units, and 

 that the scale value "0005 C.G.S. units for 1 centimetre be adopted for 

 horizontal and vertical force instruments of the Kew pattern. 



1887. T 



