28 THE SMITHSONIAN INSTITUTION. 



a current of air is allowed to pass, in order to prevent dampness. It has been 

 supplied with a set of apparatus for determining the continued variations in di- 

 rection and intensity of terrestrial magnetism. By a very ingenious application 

 of the photographic process, the invention of Mr. Brooks, of England, the instru- 

 ments are made to record, on a sheet of sensitive paper moved by clock work, 

 their own motions. First, to determine the variations of direction of the horizontal 

 magnet ; a steel bar, strongly magnetized, is suspended by several fibers 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 

 the 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 sta- 

 tionary, and consequently traces on the moving paper a simple straight line ; but 

 when the magnet is disturbed by the terrestrial perturbations, its oscillations are 

 recorded by the motion of the pencil of light in a curved or zig-zag line. 



To register the intensity or 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 attached to the ceiling, or some 

 other support. The plate is then made to revolve through an arc of a circle, until, 

 by the force of torsion, the magnet is deflected from a north and south to an east 

 and west direction. It is thus kept in a state of equilibrium between the force of 

 torsion of the threads, tending to turn its north end around still further to the 

 south ; and the magnetism of the earth, on the other hand, tending to bring it 

 back to its north and south direction. If in this position the magnetism of the 

 earth becomes stronger, it will prevail, and the north end of the needle will turn 

 toward the north. If the magnetism of the earth diminishes in intensity, the 

 force of torsion will prevail, and the same end will move toward the south. These 

 motions, as in the case of the other magnet, are recorded by a beam of light on 

 the paper surface of the revolving cylinder. But, besides the change of direction 

 of the horizontal needle, a magnet, so support^ as to be free to take any position, 

 in this latitude will arrange itself with its end dipping toward the horizon. The 

 amount of this dip, or variation, varies also in diff'erent places, and at different 

 times ; and to record these changes a bar is supported in the direction of the mag- 

 net north and south, on two knife edges, like the beam of a balance. Any change 

 which takes place in the position of a magnet thus arranged is recorded by a 

 mirror attached to the prolongation of the axes on which the bar turns. 



It is proposed to keep these instruments constantly in operation, for the purpose 

 of comparing results with observations of a similpr character in diff'erent parts 

 of the world ; and also for the purpose of furnishing a standard to which the ob- 

 servations made at various points by the Coast Survey, and the different scientific 

 explorations which are now in progress in the western portions of the United 

 States, may be referred, and with which they may be compared. 



