141 



portions of the Northern Hemisphere, it is undoubtedly known to 

 the academy that the needle is never " at rest," but continually 

 vibratino;, or changing, either to the east or to the west. These 

 observations of change have been reduced to something like regu- 

 larity. For instance, it is ascertained that somewhere about eight 

 o'clock in the morning, the needle is at its greatest eastern position. 

 At near two o'clock in the afternoon it is at its greatest western 

 position. Then it moves east again until eight or nine in the after- 

 noon. Then west until two or three in the morning, returning to its 

 eastern position at about eight o'clock in the morning of each day. 

 These hours vary somewhat in different places, and the oscillations 

 *or changes are greater in summer than in winter. In Cambridge, 

 it is said to be fifteen minutes in summer and ten in winter. In 

 Paris, it is sometimes twenty-five minutes in summer, and in St. 

 Petersburgh sixteen minutes m summer and only two in winter. 



Observations tending to show the extent of the diurnal variations 

 or changes in this State, are limited. I am informed that for a few 

 years past the President of Santa Clara College has paid some 

 attention to the subject, but the result of his labors has not yet 

 transpired. In conversation with some of the more intelligent sur- 

 veyors of the State, they appear to be aware of the existence of 

 important changes during the day, but no experiments have been 

 made by them to ascertain the range. 



Some casual observations made by me in Amador Valley, in Ala- 

 meda county, in August, 1851, between the hours of eight A. M. 

 and two P. M., showed a variation of nine minutes during that 

 period. 



The annual variation is subject, it is believed, to greater irregu- 

 larity and less system. The observations taken in tliis State, so far 

 as known to me, are meager. 



Lines run m 1850 near the old San Jos^ Mission, in Alameda 

 county, at the true variation at that time, were retraced in 1856 

 by the same person with the same instrument, at a variation show- 

 ing an average annual change of seven minutes. Lines run in 

 1852 some ten miles west of Santa Clara, in Santa Clara county, 

 now indicate a chano-e in tracing them of about two and one-half 

 minutes per annum. Lines run in the vicinity of San Francisco in 

 1852, now show changes averaging near four minutes per annum. 

 Lines run in 1851 near the base of Mount Diablo, were retraced 

 recently at a variation amounting to some two and one-half minutes 

 per annum. 



Col. Lewis, an intelHgent surveyor and mathematician of this city, 

 informs me that his observations for the last ten years go to show an 

 annual increase east in the variation, in this section of the State, of 

 about four minutes. 



Proc. Cal. Acad. Nat. Sci., April, 1862. ^^ 



