1885.] Fluctuations of Declination at Kew aud Stonyhurst. 363 



■of the best marked fluctuations during the years 1883 and 1884, both 

 large and small, abrupt and non- abrupt. 



There are two ways in which such a comparison may be made, the 

 first of these being to measure the vertical difference in the declina- 

 tion curve between the two turning points of a fluctuation. This is 

 the method which we have pursued in this investigation. It is, how- 

 ever, subject to the objection that the course of the curve between 

 two such points is not precisely a straight line, and hence that this 

 course embraces different values of abruptness. 



On the whole, however, this method as we have used it appears to 

 us to lead to definite, and we think not inaccurate, results. The 

 other method would be to compare together the simultaneous rates of 

 change of the declination at the two observatories, selecting for this 

 purpose such portions of the records as present the appearance of 

 ■constant slope, that is to say are straight lines. 



This method we have not hitherto pursued, but it is possible that 

 we may do so, and compare it with the other in a contemplated 

 future research. 



It is unnecessary to give a description of the magnetographs at the 

 two observatories, suffice it to say that both declination magnets are 

 -as nearly as possible of the same size and weight, being about 

 •5*5 inches long, 0*8 inch broad, and 0*1 inch thick. 



The scale of the arrangement is, however, different at the two 

 observatories in such a manner that at Kew 1 mm. of scale — 0*8 7\ 

 while at Stonyhurst 1 mm. of scale = 1*13'. It would thus appear 

 that equal vertical curve-differences at Kew and at Stonyhurst are to 

 •each other very nearly in the proportion of 1 to 1*3. This is the 

 proportion which we shall use in the present paper. 



For the Kew results, we are indebted to the kindness of the Kew 

 Committee ; of Mr. Whipple, Superintendent of the Kew Observatory ; 

 and of Mr. Baker, the magnetical assistant there. 



In the following table (I) we have embodied the actual results of 

 the various measurements : — 



2 b 2 



