Colonel Sabine on the larger Magnetic Disturbances, 231 



form elevation through its extent. The principal requirements are, 

 besides taking account of the depression of the sea beyond the ten- 

 depth line, to estimate the effect of the curvature of the coast to- 

 wards the mouth of the Wear, to compute the eifect of the hollow 

 of Jarrow Slake, and generally to make proper allowance for the 

 absence of matter in the valley of the Tyne. There are also some 

 small elevations to be considered. The general result is, that the 

 attraction of the regular shell of matter is to be diminished by about 

 al^th part. 

 Putting D for the mean density of the earth, d for that of the 



shell, the fraction ^I^I&J^^ is computed to be 1-00012032 

 (gravity above - ^ 



— 0-00017984 X ^. The pendulum experiments give 1-00005185. 



The comparison of these gives — = 2-6266. 



The eighth section contains a detailed account of the strata passed 

 through in sinking the Harton shaft, and the specific gravities of 

 many of the beds as determined by Professor W. II. Miller. The 

 result for the mean specific gravity is 2*50. Substituting this in the 

 equation given by the pendulum experiments, the mean specific gra- 

 vity of the earth is found to be ^'^idQ, Adverting to the excess of 

 this number above those given by the Schehallien and the Torsion- 

 rod experiments, the author remarks that it is very difficult to assign 

 the causes or the measures of error in either of the experiments, but 

 expresses his belief that the result of the present experiment may 

 compete on at least equal terms with the others. 



Feb. 14.— Dr. W. A. Miller, V.P., in the Chair. 



The following communication was read : — 



** On Periodical Laws discoverable in the Mean Effects of the 

 larger Magnetic Disturbances." — No. III. By Colonel Edward 

 Sabine, R.A., D.C.L., Treas. and V.P.R.S. 



In two previous papers bearing the same title as the present 

 (Phil. Trans. 1851, Art. V., and 1852, Art. VIII.*), the author 

 showed, from the hourly observations of the magnetic Declination at 

 Toronto and Hobarton, that the magnetic disturbances of large 

 amount, and apparently irregular occurrence, commonly called mag- 

 netic storms, are found, when studied in their mean effects, to be 

 governed by periodical laws of systematic order and regularity, and 

 to exhibit periods whose duration is, respectively, 1, a solar day ; 

 2, a solar year ; and 3, a period of about ten of our solar years, 

 corresponding both in duration and in the epochs of maximum and 

 minimum variation, to the approximately decennial period discovered 

 by Schwabe in the phsenomena of the solar spots. In the present 

 paper the author communicates the results of a similar investigation 

 into the laws of the disturbances of the two other magnetic elements 

 at Toronto, namely, the Inclination and the Total Force, derived from 

 the hourly observations of the horizontal and vertical Forces during 

 the five years from July 1843 to June 1848 ; affording, as he states, 

 * See also Phil. Mag. S. 4. vol. i. p. 498, and vol. iv. p. 232. 



