102 



Prof. B. Stewart on the Daily 



[Mar. 22, 



and latitude 50° S., which was approximately our position at Observatory 

 Bay, we obtain a secular variation of — 2' *3. We may therefore fairly 

 conclude that — 2'*5 represents the annual change with considerable 

 accuracy. 



Passing from the dip to the total force we find 11*323 to be in British 

 units the mean of three determinations from observations made on shore 

 by H.M.S. 1 Erebus ' and ' Terror.' If, now, we apply the correction 

 + 0*1 for the change from Christmas Harbour to Royal Sound, the result 

 is still somewhat less than the mean of the observations taken near the 

 eastern extremity of Kerguelen during the epoch 1840-45. Adopting 

 11-423 as the mean value for 1842-45, and 11-143 for 1875, we obtain a 

 secular diminution of 0*0086 in this element of terrestrial magnetism. 



The annual increase of the declination will be + 7'*0, if we take the 

 approximate value of 32-0 W. from the map of Sir E. Sabine as repre- 

 senting the declination for the epoch 1842-45. 



IV. " On the Variations of the Daily Range of the Magnetic 

 Declination as recorded at the Kew Observatory. 93 By 

 Balfour Stewart, LL.D., F.R.S., Professor of Natural 

 Philosophy at the Owens College, Manchester. Received 

 February 28, 1877. 



1. The daily range of the magnetic declination at any station may 

 perhaps be regarded as a convenient representative of the magnetic 

 activity of the place. For while a thorough discussion of the diurnal 

 magnetic changes must embrace along with the declination the two com- 

 ponents of the force, yet, as regards such daily ranges, the declination 

 gives results which are not only more prominent but also more easily 

 procurable and subject to fewer uncertainties than similar ones for the 

 other two elements. 



In estimating the daily range of the magnetic declination, as recorded 

 at the Kew Observatory, I have excluded the disturbed observations, con- 

 ceiving that by so doing a better indication of the true magnetical activity 

 of the place would be obtained than by including them, inasmuch as they 

 follow a very different set of laws from that of the well-known diurnal 

 declination-range. The disturbed observations have been separated by 

 the method of Sir E. Sabine, those being rejected as disturbed for which 

 the measurements on the photographic curve are 0*150 inch either above 

 or below the mean value for that month and hour, one inch denoting 

 22'*04 of angular change. The daily ranges are here given in inches, and 

 they denote the differences between the greatest and least values of each 

 day's hourly tabulations from the curve, disturbances being excluded. I 

 am indebted to the kindness of the Kew Committee for giving me the 

 daily ranges herein discussed, extending from the beginning of 1858 to 

 the end of 1873, thus embracing in all sixteen years' observations. 



