Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/274

Rh zoo SCOPY) ; some are produced by the permanent gases, others by aqueous vapour, which is always present, though in variable amount. The absorption -spectrum of water- vapour has been minutely studied and carefully mapped by Janssen and by Cornu. In Angstrom's table of normal solar spectra there are numerous groups of lines which appeared most conspicuous when the sun was close to the horizon, and many of these are coincident with the absorp- tion-lines of water-vapour. They are found principally in the red and yellow, and the main group is seen a little to the red side of the D line. In small spectroscopes the water-vapour lines appear fused together into a band on the red side of jD, or even as a mere widening of that line. There are several variable bands in the spectrum which come to a maximum of intensity when the sun is on the horizon. These are due to absorption by the permanent gases ; the one which in a small instrument appears to separate the yellow from the green is frequently mistaken for the water-vapour band, and this is the cause of many incorrect " rainband predictions." In 1872 Professor Piazzi Smyth noticed a change in the water- vapour lines before and after a sirocco in Palermo, and the same phenomenon was brought before him very strikingly in France before great rains in 1875 (Edin. Ast. Obs., vol. xiv.), when he named the main group of water-vapour lines the " rainband." The rainband may be observed with any spectroscope, but direct- vision instruments of medium size are most convenient. It is im- portant that the spectroscope should have as great dispersion as possible and good definition, especially at the red end. To make an observation the slit, which must be kept perfectly clean, should be narrowed down until the spectral lines are sharp and clear. The instrument should be carefully focused to get the maximum absorp- tion effect ; but all observers lay stress on noting whether the intensity of the band decreases rapidly or gradually as the altitude is increased. When a dark band is observed at the horizon and at the zenith, heavy rain is almost certain to follow immediately. It is of little importance whether the sky be clear or covered with high clouds at the point of observation ; low clouds or haze make the result untrustworthy by shortening the line of sight, thus reducing the strength of the band and equalizing it in all directions. The utility of the spectroscope in meteorology depends on its power of investigating the hygrometrical conditions of the whole slice of atmosphere looked through, and it affords a means of ascertaining the difference of humidity in different directions. The hygrometer only indicates the state, as regards moisture, of the few yards of air surrounding it. The great difficulty in the way of obtaining accurate results with the rainband spectroscope has been the mental scale of comparison employed by most observers. Although some have found it easy to estimate the intensity of the baud from to 20, it is beyond the power of many to describe it in figures even from to 5. Professor Piazzi Smyth noted its strength relatively to that of the dry-air band between the green and yellow. The fixed solar lines E, b, and F have been used as a closer approxi- mation to a scale in instruments not powerful enough to separate the rainband from the D line. This compound line appears to vary from something less intense than E in very dry weather to something considerably darker than F when rain is imminent. Seven degrees can be discerned, and these may be represented by using the sign = to mean "of equal intensity with, > to mean "darker than," and < "less dark than," as, < E, = E,> E < b, = b, > b < F, = F, and > F. When the thin solar lines in the green are seen very distinctly there is less probability of rain falling than when they are indistinct or invisible. The following table gives an idea of the intensities corresponding to rain-probabilities at Edinburgh.

It appears that the average percentage of fulfilments of ro- of "rain " and "no rain," made for a period of twelve he predictions lours after one observation in the morning, which may be expected in Scotland is about 75. In less variable climates, such as those of the south of Europe and parts of the United States, a much higher degree of accuracy has been attained. The precise strength of rainband which corresponds to the probability of a fall of rain within a definite time depends on the temperature and also on the place of observation ; in every case it must be determined by the observer for himself. Very dark rainbands are found to precede rain in more than 95 per cent, of the cases everywhere ; entire, or almost entire, absence of the band presages a dry day with equal prob- ability. With a spectroscope powerful enough to split 1) a mental numerical scale must be used in default of a suitable microniri, i. and by practice the observer will be able to draw up a table for its conversion into probabilities of rain. The production of an artificial absorption -line the intensity of which could be varied by known degrees suggested itself to more than one observer as the principle for a rainband-micrometer. A wedge of didymium glass comes very near success in this direction, but the absorption-line is awkwardly situated. Professor Cook of Dartmouth College, the leading meteorological spectroscopist in the United States, has constructed a micrometer for a spectroscope of sufficient power to separate the rainband into lines. A silk fibre is fixed to a frame capable of being moved to and fro in the tube of the spectroscope by a micrometer screw. When brought into focus the fibre appears as two sharp lines, which become fainter and wider as it is withdrawn. When the lines appear of equal intensity with the most prominent line of the rainband (a of the D group in Janssen's map) the micrometer is read ; forty shades of intensity may be indicated by it. The unit proposed for graduating such micrometers on a uniform scale is the intensity of the faintest and least refrangible of a group of three lines in the red (w. I. 6207) of the spectrum produced by a 1 centimetre column of the gas given off by heated lead nitrate (mixture of nitrogen peroxide and oxygen) at 25 C. and 760 mm. The spectroscope when reinforced by this micrometer has been found to indicate with unfailing accu- racy the existence in certain parts of the sky of banks of invisible cloud, which become visible when a fall of temperature and other necessary conditions allow the vapour to condense. In settled weather such masses of vapour are not to be found, the micrometer readings being the same in all directions at the same altitude. Sometimes the rainband grows gradually darker for several days before a period of steady and long-continued rain, while sudden violent showers may give very short notice. The spectroscopic history of a thunderstorm observed on 9th June 1884 by Professor Cook is extremely interesting. During its approach the water-vapour line observed at 10 north of the zenith towards the storm was darker by ten degrees of the scale than that observed 10 south of the zenith, although to the eye the clouds presented exactly the same appearance at both places. On this occasion the strength of the line varied as follows : Hour. At zenith. At 30. At 20. At 10. At horizon. 8 a.m.. 13 21 32 43 45 9 . 14 23 34 44 46 10 . 14 25 36 45 47 11 . 15 27 37 46 48 11. 45 a. m 50 Rain commenced 11.30. 12 noon. 32 Rain ceased 12.10. 12. 45 p. m 40 ,, recommenced 12.20. 1.30 15 Rain ceased at 1. Total rain- fall, 1-6 inches in 1J hours. Some relation has been traced between the variations of the rain- band and the appearance of aurora, but this matter is not yet fully investigated. (H. R. M.) RAINBOW. See LIGHT, vol. xiv. p. 595 sq. RAINGAUGE (PLUVIOMETER, HYETOMETER, UDO- METER). The value of the measurement of rainfall (see METEOROLOGY) has long been understood, although it is only within the last hundred years that trustworthy results have been obtained. Marriotte is claimed as the originator of the raingauge in 1677. The simplest form is an open vessel of uniform diameter exposed to the rain, in which the depth of water collected during any interval of time may be measured. In order to reduce evaporation the mouth of the gauge is usually a funnel of the same diametei as the vessel ; and some means, such as an external narrow glass tube graduated in inches and parts to show the height of the water inside, or a float bearing a graduatec rod, or, in more delicate forms, a movable scale which ma? be set by a vernier to the surface of the water, is adoptee to facilitate measurement. Raingauges on this plan can not be very accurate ; their one advantage is that the ar of the collecting surface does not require to be known The disadvantages are that so much water is required fr wet the sides of the vessel as to make the instrument rea too low when a side tube is used, and the tube is liable t