=Forecasting from Halos.= The halos that sometimes surround the sun or the moon indicate the coming of precipitation to the extent of making manifest the presence in the upper air of large quantities of vapor of water in a congealed state. When the vapor of water cools quietly in the laboratory it frequently forms minute spheres of water, which, strange to relate, may remain liquid all the way down to zero and below; but if touched or jostled they instantly turn to ice, in the form of spiculæ, or needles; they are simply hexagonal slender prisms capped by hexagonal pyramids. These needles rotate or spin about as they fall. The geometrical relations of the facets of the crystals to the axis of rotation and to the line along which they fall are a complex problem in optics. Suffice to say that the observer, looking through a filmy cloud of such crystals, would see in one part of the sky a halo, in another part an arc of light, and in other directions bright spots like the sun, all of them arranged symmetrically with regard to the sun and the observer’s zenith. A lunar halo is a large ring concentric about the moon. A secondary halo surrounds the first. Mock suns or mock moons may appear coincident with solar or lunar halos. The ice prisms through which one sees the phenomena both refract and diffract the light as it passes through the cloud and by partly decomposing the rays render visible a part of their elementary colors. The red is on the inside, next to which is a little yellow or green, with bluish white on the outside. In coronas, which are much smaller, the red is on the outside. A detailed description of these phenomena may be found in Moore’s “Descriptive Meteorology” (Appleton).