20 EIGHTH ANNUAL REPORT OF 



day, others upon the season of the year, others again upon longer 

 periods of time. 



It also varies in its direction at different places. Between the At- 

 lantic and Pacific coast, or, for example, between Massachusetts bay 

 and the mouth of the Columbia river, there is a variation of upwards 

 of twenly-four degrees ; but this variation is not constant even at the 

 same place, but changes from year to year. With these changes it is 

 necessary that the navigator should be familiar. It therefore becomes 

 a matter of national importance that observations of these phenomena 

 should be made at as many places and those as widely separated from 

 each other as possible. 



The Smithsonian Institution has endeavored to advance this branch 

 of knowledge, by importing at different times, and at considerable ex- 

 p^ise, four entire sets of apparatus, besides separate instruments, for 

 determining the direction and intensity of the magnetic force. 



Tliese instruments have been lent to observers, and in some cases 

 sold to the government ibr the use of exploring parties, and have done, 

 or are now doing, good service, in adding to the stock of facts which, 

 by the process of induction, are to yield a knowledge of general laws. 



It will be recollected that an appropriation was made at the last ses- 

 sion of the Regents for suppl3nng magnetic instruments to the Grinnell 

 Expedition. These were procured from London, were given in charge 

 to Dr. Kane, and, we trust, are at this time revealing to that intrepid 

 explorer the fitful and mysterious changes of the magnetic force. 



During the past year, a magnetic observatory has been erected within 

 the grounds of the Smithsonian Institution. It principally consists, to 

 insure an ecpable temperature, of an under-ground room, enclosed 

 within two walls, between which a current of air is allowed to pass in 

 order to prevent dampness. This observatory has been supplied 

 with a set of apparatus for determining the continued variations in 

 direction and intensity of terrestrial magnetism. By a very ingenious 

 application of the photographic process, the invention of Mr. Brooks, 

 of England, the instruments are made to record, on a sheet of sensi- 

 tive paper moved by clock-work, their own motions. 



First, to determine the variation of direction of the horizontal magnet : 

 a steel bar, strongly magnetized, is suspended by several fibres of un- 

 twisted silk, so as to have perfect freedom of motion in the horizontal 

 plane, and from a gas light, kept perpetually burning, a single ray of 

 light is thrown upon a concave mirror permanently attached to the 

 magnetic bar, and consequently partaking of its movements. This 

 ray of light is reflected and brought to a focus at the surface of 

 a revolving cylinder, moved by clock-work, on which the photo- 

 graphic paper is placed. When the magnet is at rest, the pencil of 

 light is stationary, and consequently traces, on the moving paper, a sim- 

 ple straight line ; but when the magnet is disturbed by terrestrial per- 

 turbations, its oscillations are recorded by the motion of the pencil of 

 light in a curved or zigzag line. 



To register the intensity of strength of the magnetic force, another 

 bar magnet is suspended by two parallel silk threads about an inch 

 apart, descending from two hooks fastened to the under side of a plate 



