Page:Popular Science Monthly Volume 12.djvu/476

460 In addition to the concave mirror, each magnet carries a plane mirror to receive the reflection of a scale attached to the back of a small telescope. The telescopes are mounted upon the stands of the recording instruments, and for convenience of observation are provided with diagonal eye-pieces. On looking into either telescope one sees its scale reflected in the plane mirror carried by one of the magnets. When the mirror turns with the swing of the magnet, the scale appears to the observer to traverse the mirror's face. At the beginning and again at the end of each trace a record is made of the division of the scale then covered by the cross-wires of the telescope. Knowing the times at which each trace was started and stopped, and the readings of the scale at those times, it is easy to divide off the paper into spaces corresponding to the hours of the day and into other spaces at right angles to these corresponding to divisions of the scale. An exact record is thus made of all magnetic variations.

Particular interest attaches to magnetic observations on account of the way in which the magnetic state of the earth seems to be influenced by the position of the sun, and to a slight degree by the position of the moon; also from the connection between auroral displays and magnetic variations, curves representing the frequency of either agreeing quite closely with curves representing the area of the sun covered by spots. It is well to remark that the curves representing magnetic variations and auroras lag about six months behind those representing the sun-spot variations. The sun-spot area seems in some way to depend upon the position of the planets. Not only is the earth's magnetism thus, seemingly at least, influenced by the sun-spots, but also some of the phenomena of the weather. These last are, of course, in general, masked by local disturbances; but, lately, a very remarkable agreement has been shown to exist between certain magnetic and barometric traces. Investigations into the causes of magnetic variations and the laws under which these variations occur are made by officers of the United States Coast Survey; and to the head-quarters of this survey, at Washington, are forwarded, each month, the traces obtained at Madison.



O form the seed seems to be the chief end of the plant. When in the vigor of its own maturity, and when receiving the sun's strongest rays and the earth's richest nourishment, the plant gathers 