Page:The American Cyclopædia (1879) Volume XIV.djvu/203

 RAINBOW 1ST been made at about 1,200 stations, showing that the slight general variations in the rain- fall throughout the country have somewhat of a periodical nature ; thus along the seaboard from Maine to Virginia, as also in New York, and in the Ohio and Mississippi valleys, there has been an increase (amounting however to scarcely 1 per cent.) in the average annual precipitation during the last 50 years ; on the southern Atlantic coast it appears to have been on the decrease. The following table, condensed from those of Schott, gives the relative rainfall by decades for several sections of the United States : DECADE. Eastern and Middle States. New York State. The Northwest. The Ohio Valley. The Southwest. The Gulf States. The South Atlantic States. California. 1810-M9 0-933 l820-'29 0-971 1880-'39 0-981 0-933 0-926 0-955 1840-'49 0-999 0-978 0-969 1-048 1-067 1-000 1-068 1850-'59 1-050 1-028 1-031 1-009 972 0-962 0-974 1-211 1860-'69... 1-068 1-057 1-030 980 1-032 1-066 Both the British and American series there- fore unite in showing that during 60 years there has been no appreciable change. For further details on the subject of rain, see Wojeikof, Die atmospJidrische Circulation, appendix No. 38 to Petermann's Geographi- echen Mittheilungen (Gotha, 1874). For in- formation relating to the United States, see the above cited "Tables of Rainfall." With re- gard to the rainfall in Great Britain, see the annual volumes of " British Rainfall," by G. J. Symons, which contain every variety of in- formation on this subject, including the actual measurements at 1,500 stations and numerous special investigations into sources of error. RAINBOW, an arch of concentric colored bands, visible usually on a portion of sky overspread with falling rain drops, and always on that side of the observer opposite to the place from which the sun or moon is shining at the time. When the field of falling drops is large, and the illumination thrown on it is bright, a second bow, exterior to and concen- tric with the first, appears. The inner, or most usual, is termed the primary, the outer the secondary bow. Each shows the same colors, and in the same succession, as those obtained in decomposing a beam of sunlight by means of a dispersing prism of glass ; but in the two bows the colors lie in opposite or- der; in the primary the red is outermost, in the secondary innermost. The primary is al- ways the brighter, and decidedly the narrow- er. When the light is abundant, this bow is often accompanied by successive bands of red and green, lying just within it or overlapping its violet edge, concentric with it, but extend- ing through parts of its course only, and es- pecially where it nears the horizon ; these are called supernumerary bows. The common centre of the two bows is always in the direc- tion of the antisolar point ; so that, of course, the rainbow rises at the same rate as the sun declines, or declines if the sun is rising. The conditions requisite to produce the rainbow have been in a general way understood from an early period, though its causes were not. The earliest known attempt at an explanation of it is that of Aristotle. He observed that from a glass globe filled with water, and set in the sun, certain colors were always returned at certain angles with the course of the sun's beams ; and he properly explained the circular form of the bow, by saying that if the sun- beam passing through the observer's eye be taken as an axis, and the globe be revolved round this axis, and at the same distance from it in all parts of its course, the same colors, preserving their angle with the direction of the sunbeams or of the axis, would be visible through all parts of this course ; and hence it followed that a rainbow would result if there were globes enough, and so placed as to reflect colors at the same time from all parts of an arc of such a circle. The colors were sup- posed to be merely reflected from the globe, or (in the sky) from the drop of water, until Fleischer of Breslau (1571), concluding that reflected light does not give colors, stated as a consequence that the rays must enter the drops. Of the light falling on the presented side of the drops, of course part will be reflected, but another part will enter and be refracted at the same time ; striking on the inner opposite sur- face of the drop, part of this beam will emerge and escape, while another part will be re- flected ; and on again striking the side of the drop toward the spectator, though a portion of this residue of the first beam undergoes a second reflection, another portion emerges, again refracted, and, if at a proper angle, then passes to the eye. Kepler agreed in this view, but erred in supposing the entering light to be that of rays grazing or tangent to the upper sides of the drops. Antonio de Dominis, in 1611, carefully repeated the experiments with the glass sphere filled with water, showing in sunlight very vivid colors to a great distance, and each at an angle of its own. Descartes showed: 1, why there must be on the illumi- nated field of falling drops a circular belt of col- ors bright enough to be seen, and always of a definite diameter ; and 2, that the colors are in separate bands or stripes in this, because they are not equally refracted. He gave the reasons why the colors must be just where they were, and in bands just so broad, if they all appeared ; he could not tell why they must all appear. This element Newton supplied, when he dis- covered (1666) that sunlight is decomposable