Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/275

Rh R A I R A I 257 Fig. 1. Fig. 2. FIG. 1. Glaisher's rain and be broken in frost ; when a float and rod are employed, the rod projecting above the funnel catches rain and the resulting reading is too high. Almost all raingauges now used have a circular funnel of known diameter, which con- ducts the rain-water to a receiver, from which it may be poured into a special narrow measuring-glass so graduated that what would cover a space of the area of the funnel to the depth of 1 inch fills a portion of the glass large enough to be easily graduated into 100 parts. The funnel may have any diameter from 3 to 24 inches without introduc- ing a greater discrepancy than 1 or 2 per cent, of the amount of rain collected, but 5 and 8 inches are the dia- meters usually employed, and the measuring -glasses are graduated accordingly. Advantage is sometimes taken of the fact that for a funnel 4' 697 inches in diameter 1 fluid ounce of water collected represents one-tenth of an inch of rain, since the area is 17' 33 square inches, and a fluid ounce at 60 Fahr. contains I 1 733 cubic inches of water. The best form of instrument, Glaisher's rain and snow gauge recommended by the Royal Meteoro- logical Society, is a cylindrical copper vessel 8 inches in diameter and 18 inches high, in which the funnel is placed about halfway down (see fig. 1 ). The Scottish Meteorological Society largely employ Howard's raingauge, a plain glass bottle holding about half a gallon and provided with a long 5 -inch copper funnel, which has a collar fitting over the neck of the bottle to prevent rain from being blown in laterally by the wind. In some forms the funnel leads to a long glass tube divided into inches, tenths, and hundredths of rainfall. Mr Symon's storm raingauge on this prin- ciple is intended to be read from a distance, and is only graduated into tenths of an inch. The water col- lected by a raingauge may be weighed snow gauge, instead of measured, but the latter FlG - 2. -Graduated measuring process being much more simple is glass, always adopted, at a slight expense of accuracy, however, as the variation of volume with temperature is not taken into account. Precipitation and evaporation being complementary phenomena, an atmometer or evaporation-gauge ought, strictly speaking, to ac- company each raingauge. But none of the instruments yet devised can be regarded as satisfactory, accordingly a number of devices have been introduced to calculate or to minimize the evaporation from raingauges. Dr Garnett in 1795 proposed to use two gauges of unequal size, and recently Prof. Michie Smith has introduced a simplification by making the area of one gauge exactly double that of the other. If the evaporation is the same from each, the difference between the readings of the two gauges gives the true rainfall in the smaller. If A be the area of the funnel in the smaller, 1A that of the funnel in the larger, V a certain volume of water placed in each gauge, E the evaporation, and R the inches of rainfall, then V+ 1AR - E - ( V+ A R - E) = AE, however V and E may vary. The simplest and best method is to use a funnel terminating in a long straight tube, which reaches almost to the bottom of the receiving vessel. Gauges have been constructed for experimental purposes always to face the wind, and with openings capable of being fixed at any angle. For use at sea they may be swung on gimbals ; but when so employed the record must be sup- plemented by the readings of a hydrometer so as to detect and allow for any admixture with sea-spray. Self-registering and self- recording raingauges, as frequently used in meteorological observa- tories, are constructed on two leading types. In Hermann's "hyetometrograph," 1789, a fixed funnel conducts the rain into one of twelve glasses placed on the circumference of a horizontal wheel, which is turned by clockwork so that each glass remains under the funnel for one hour. In Stutter's more recent instru- ment the receiving funnel delivers into a smaller funnel, which has a sloping tube and is carried round by clockwork so as to remain for one hour over each of twenty-four fixed glasses arranged in a circle. The second kind of self-registering instrument produces a continuous record of rainfall, indicating the hour of commencement and close of each shower, the amount of rain that has fallen, and the rate at which it fell. In Beckley's " pluviograph " a pencil, attached to a vessel which sinks as it receives the rain, describes a curve on a sheet of paper fixed round a rotating cylinder ; when full the receiver empties itself by means of a siphon and the pencil is carried rapidly upwards, describing a straight vertical line. The higher a raingauge is placed above the ground, or rather above a broad flat surface, the smaller is the rainfall registered, as the following figures indicate : Height of funnel above ground in feet.. 01 10 20 40 200 Rain registered (average) 1'07 TOO 0'93 0'90 0'70 0'58 When the mouth of the gauge is on or within a few inches of the ground the insplashing of raindrops increases the amount of water. Minute rain-spherules, which usually float in horizontal or oblique planes, are most numerous near the ground, where consequently they coalesce to form regular drops which fall into the funnel. The raindrops also increase slightly in size by condensing moisture as they fall. But the greatest effect is probably produced by wind, which forms eddies round high and isolated objects, thus more or less interfering with the fall of rain into the gauge. It is obvious that all raingauges intended for comparison should be fixed at the same height, and in Great Britain the standard distance of the mouth of the funnel from the ground or from a broad flat surface is one foot. The situation of a raingauge should be perfectly open, especially in the direction of the prevailing rainbringing winds. In measuring rain it is essential to see that the funnel is not indented or deformed in any way, and that the collecting vessel is inaccessible to air or rain except through the funnel. The temper- ature of rain as it falls should be observed whenever it is possible to do so. The amount of solid matter collected in the raingauge should be ascertained and recorded as bearing on Mr Aitkeu's theory of rain (see EVAPORATION), and it should be examined micro- scopically for volcanic and cosmic dust. (H. R. M.) RAIPUR, a district of India, in the Chhatisgarh division of the Central Provinces, lying between 19 48' and 21 45' N. lat. and 80 28' and 82 38' E. long., with an area of 11,855 square miles. It is bounded on the N. by Bilaspur, on the E. by Sambalpur and Patna, on the W. by Balaghat, Bhandara, and Chanda, and on the S. by Bastar and Jeypur. The district spreads out in a vast plateau closed in by ranges of hills branching from the great Vindhyan chain. It is drained by the Seonath and the Karun rivers, which subsequently unite and form the Mahanadi. Geologically the country consists in the hilly tracts of gneiss and quartzite ; the sandstone rocks in the west are intersected with trap dykes. Iron ore is abun- dant, and red ochre of high repute is found. In the interior the principal strata are a soft sandstone slate (covered generally by a layer of laterite gravel) and blue limestone, which crops out in numerous places on the surface and is invariably found in the beds of the rivers. Throughout the plains the soil is generally fertile. The climate is generally good ; the mean temperature is 78 and the average rainfall about 49 inches. The population of Raipur in 1881 was 1,405,171 (males 696,242, females 708,929). By religion 856,492 were Hindus, 14,991 Moham- medans, and 821 Christians. The only town with a population exceeding 10,000 is RAIPUR (see below). Attached to the district are four feudatory states, viz., Chhuikhadan (with 32,979 inha- bitants), Kanker (63,610), Khairagarh (166,138), and Nandgaon (164,339). Their combined area is 2658 square miles. Of the total area under British administration only 3636 square miles are cultivated, and of the portion lying waste 4337 acres are returned as cultivable. The staple crop is rice ; other crops are wheat, food-grains, oil-seeds, and cotton. The commerce of" Raipur is of quite recent creation, for under the Mahrattas the transit dues that were levied prevented its development. The exports consist mainly of grain, cotton, and lac, while metals constitute the chief import. The gross revenue of the district for 1883-84 amounted to about 89,829, of which the land yielded 64,871. Raipur was governed by the Haihai-Bansi dynasty of Ratanpur for many centuries until their deposition by the Mahrattas in 1741. The country was then already in a condition of decay, and soon afterwards it relapsed into absolute anarchy. In 1818 it was taken under British superintendence and made rapid progress. It fell with the rest of the Nagpur dominions to the British Government in 1854. Raipur suffered but little during the mutiny. EAIPUR, chief town of the above district and head- quarters of the Chhatisgarh division of the Central Pro- vinces, is situated in 21 15' K lat. and 81 41' E. long., on a plateau 950 feet above sea-level. In 1881 its population amounted to 24,948 (12,447 males and 12,501 females). The modern town dates from 1830, and carries on a flour- ishing trade in grain, lac, cotton, &c. XX. 33