RAIN. 674 KAINBOW. face cf the sky is covered with <lirk clouds which pour down prodigious quantitie-i of rain. Toward evening the clouds disappear; the sun sets in a dear skv. and the nights are serene and fine. In latitudes 25° X. to 35° N. on both Atlantic and Tacifie there is a region of northeast trade winds in which the rainfall is comparatively light and occurs equally in all parts of the earth. The details of the distribution of rainfall in the I'nited States are given by Professor A. J. Henry in his "Rainfall of the United States," Wcni'licr Biircnu BiiUctm D, 1S97, and especially in the article, "Rainfall and Charts of Rainfall." in the iloiilhhi ^yeat^ler Revieic for April, 1002 (Washington, 'D. C). RAIN-BIRD. Any bird popularly believed to foretell rain. Many of these birds are cuckoos. Both of the North American species have this reputation, as also have the black anis of Mexico and southward, which are fre- quently called 'rain-crows.' In the middle west- ern portion of the United States the bird com- monly called 'rain-crow' is the j-ellnw-ljillcd cuckoo. The East Indian cuckoos, called 'koels' (q.v. ), have the same reputation, and are com- monly known in India and the ;Ialay countries as 'rain-birds' and 'foretellers.' To what extent this repute is justified is a question admitting of discussion. Birds, as well as other animals, are no doubt sensitive to changes in the tempera- ture and humidity of the air. and some may be quick to recognize that certain of these changes portend disagreeable weather. The immediate efl'ect or their anticipations may lead tliem to make outcries indicative of discomfort or alarm. A great variety of information and folk-sayings on this subject is given in Dunwoody's "Weather Proverbs," Signal Service Notes, No. ix. (War Department, Washington, 1883). For Oriental superstitions, see Ihis (London, 1879). RAINBOW (A.S. liegnloga, renhoga ; OHG. rcgan-hogo : Ger. Ilegenhogcn, from licgeii, rain, and hoga, bow). The term applied to the arc of prismatic colors which at times is seen when the sun or moon is shining while it is raining. It alwaj's is seen in the part of the heavens opposite to the sun, and is high Avhen the sun is low, and low-, near the horizon, when the sun is high. It is a short arc, or a complete one, resting on the earth at each end, according to the extent of the rain. Sometimes a second concentric bow is seen with the colors reversed. Both are due to the reflection and refraction of the rays of the sini in the drops of rain. If the 'parallel lines S, Fig. 1, represent rays coming from the sun and tailing u])nn the drop of water, the centre of which is 0, it is possible to determine the path of each ray by applying the simide law of refraction. For example: the ray SB will be retracted to A, reflected to C' and finally refracted out to T. The parts of the ray which are reflected out at B, refracted out at A, or reflected in at C, need not be con- sidered, as they do not contribute to the phe- nomenon. It may be shown that when the arcs BA and AC are equal, then the angle between SB and CT is greater than for any other case. In other words, when the ray passes through the drop synuiietrically. the final direction of the ray is at the greatest angle with the original direction, and also that a greater proportion of the total light falling on the drop is sent out in this direction than in any other. The light which falls farther out on the drop than B is mostly reflected ofT the drop, and that which falls nearer than B enters the drop, but Fig. 1. mostly passes out at the back at A. Inasmuch as the index of refraction is different for the different colored lights, it follows that the angle between SB and CT must be different for the different colors, less for violet than for red. An eye at T, Fig. 2, looking toward the drop, would see considerable of the light coming in the direction CT, but an eye at Ei would only see a little light corresponding to the ray SBjAnC.T,, and an eye at E. would see no light from that drop. If we imagine the whole of Fig. 2 revolved upon the line TS' as an axis, then Fig. 2. the ray SB becomes a cylindrical shell of rays^ the drop becomes a circular are of drops, and the ray CT becomes a conical shell of rays, which, seen by the eye at T, appears as a ring of light against the clouds as a background, having an angular radius equal to CTS'. If the red rays are considered this radius would be altout 42° 22', and for the violet rays 40° 35', and for the intervening colors it would have corresponding values. Thus the ring of violet would appear the smallest and the red the largest, with the
