In addition to its ocean magnetic surveys the _Department of Terrestrial Magnetism_ also carries on extensive land surveys in all parts of the globe. In 1919 special expeditions were sent out by the Department to observe the total solar eclipse of May 29th at stations distributed over the entire zone of visibility of the eclipse and immediately outside. At Dr. Bauer's station in Liberia the total phase was visible in a cloudless sky for more than six minutes, which is very close to the maximum length of phase that can possibly be observed. Unmistakable evidence was gathered at all stations of an appreciable variation in the earth's magnetic field during a solar eclipse, which variation is the reverse of that causing the daylight portion of the solar diurnal variation of the needle. In addition to the magnetic survey work on land and sea which is the chief work of the _Department of Terrestrial Magnetism_, atmospheric-electric observations are carried on continually on land and sea and experiments have been carried on at Langley Field, Va., lately, in the development of methods and instruments for determining the geographical position of airplanes by astronomical observations. There has also been recently formed under this department a _Section of Terrestrial Electricity_. The cause of the earth's magnetic field is still one of the greatest unsolved problems of astro-physics. The theory that has been advanced by Schuster that all large rotating masses are magnets _as a result of their rotation_ has received considerable attention from astrophysicists, and attempts have been made to prove this experimentally. It has been found that iron globes spun at high velocities in the laboratory do _not_ exhibit magnetic properties. This may mean simply that the magnetic field is too weak to be detected in the case of a comparatively small iron sphere spun for a limited period under laboratory conditions. It must be remembered that the earth has been rotating rapidly on its axis for millions of years and is, compared to terrestrial objects, an extremely large mass. Yet it has been shown that as a whole our earth is an extremely weak magnet, and that if it were made entirely of steel and magnetized as highly as an ordinary steel-bar magnet, the magnetic forces at its surface would be a thousand times greater than they actually are. If it is true that all rotating bodies are magnets, then all the heavenly bodies, planets, suns and nebulæ are surrounded by magnetic fields. We know nothing to the contrary. In fact, we know this to be true for the earth and sun, and strongly suspect that it is so in the case of the planets Jupiter and Saturn. When we understand more about the properties of matter, the nature of magnetism, as well as of gravity, may be revealed to us. XXV SOME EFFECTS OF THE EARTH'S ATMOSPHERE UPON SUNLIGHT