Page:Popular Science Monthly Volume 68.djvu/561

Rh in the earth. This terrestrial magnetism is evidenced through changes in the compass needle, in frequent magnetic 'storms' and in the beautiful northern lights. The sun spots are thus the seat of some great solar activity of whose exact nature we are at present not sure. Moreover, the sun is not sending to the earth a constant amount of heat. Very recent observations in Washington show that these solar fluctuations are followed very closely by variations in temperatures all over the earth. Whether these newly discovered variations in the sun's heat are connected with the spots on the sun, or not, it is too early to say, but it may not be outside the bounds of possibility to be able in the future to forecast the great variations in our earthly temperatures from observations on the sun. We realize, then, the importance of

these observations on the sun, and it is for the solution of problems such as these that eclipse expeditions are sent out.

To investigate the corona, photographs are taken of all sizes. The diameter of the sun on the photograph depends on the length of the camera, the greater the focus the larger the sun. At Daroca the largest camera used was forty feet in length, which gave an image of the sun four inches in diameter. In photographing, the lens can be mounted pointing directly at the sky, or else the whole instrument can be placed horizontally and light be reflected into it by means of a mirror. We adopted the latter way as being the easier.

The location of the eclipse camp was half a mile south of the town, in the midst of a beautiful, fertile valley. From there, while we