Excerpt from Discussion of the Magnetic and Meteorological Observations, Vol. 1: Made at the Girard College Observatory, Philadelphia, in 1840, 1841, 1842, 1843, 1844, and 1845; Investigation of the Eleven Year Period in the Amplitude of the Solar Diurnal Variation and of the Disturbances of the Magnetic Declination It is proposed in the present paper to investigate the law of the eleven year period, or as it is more frequently called, the decennial period, there being yet an uncertainty as to its precise length. It is supposed to have some direct or_ indirect connection with the solar spot period, which, according to late investigations by Prof. R. Wolf,2 is said to exhibit corresponding disturbances. The discussion is a contribution towards the determination of the epoch of the occurrence of a minimum (as to number and magnitude) in certain phases of the magnetic variations and disturbances, corresponding to a minimum in the solar spot period. The method of reduction is substantially the same as that adopted by General Sabine, and explained in his discussion of the Toronto and Hobarton3 observations. This longitude depends on that of Cambridge observatory, for which 411 44 has been adopted. About the Publisher Forgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.com This book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully; any imperfections that remain are intentionally left to preserve the state of such historical works.