RAIN-GAUGE. 



RAIN-GAUGE. 



630 



of York, but the values of its variable co-efficient are very different, a 

 subject which he next proceeds to investigate, with reference also to 

 Arago's observations at Paris. He finally obtains a remarkable and 

 continued accordance between the co-efficient fixed by observation 

 and those derived by two methods from a very simple view of the 

 condition of the air as to heat and moisture, which appears to him 

 decisive of the question as to the general cause of the variation of the 

 quantity of diminution of rain at any one height above the ground. 

 The observations strictly warrant the conclusion that the ratio of 

 diminution at different heights is constant throughout the whole year. 

 Professsor Phillips, therefore, offers the hypothesis, as a matter of very 

 probable inference, " That the whole difference in the quantity of rain, 

 at different heights above the surface of the neighbouring ground, is 

 caused by the continual augmentation of each drop of rain from the 

 commencement to the end of its descent, as it traverses successively 

 the humid state of air at a temperature so much lower thnn that of 

 the surrounding medium as to cause the deposition of moisture upon 

 its surface. This hypothesis," he remarks, " takes account of the 

 length of descent, because in passing through more air more moisture 

 would be gathered ; it agrees with the fact that the augmentation for 

 given lengths of descent is greater in the most humid seasons of the 

 year ; it accounts to us for the greater absolute size of ram-drops in 

 the hottest months and near the ground, as compared with those in 

 the winter and on mountains ; finally, it is almost an inevitable conse- 

 quence from what is known of the gradation of temperature in the 

 atmosphere, that some effect of this kind must necessarily take place." 



Mr. Howard has observed that, in this country, when the moon has 

 south declination there falls but a moderate quantity of rain, and that 

 the quantity increases till she has attained the greatest northern 

 declination ; and on some such results of observation the popular 

 opinion that there is a connection between the alternations of rain and 

 fair weather and the changes of the moon may be founded. The 

 vexed question of the influence of the moon upon the weather appears 

 to have received at length at least an approximative solution. Hum- 

 boldt, in his ' Personal Narrative,' mentions as a phenomenon well- 

 known to the pilots and seamen of Spanish America, the tendency to 

 disappearance of clouds under the full moon. This meteorological fact 

 has also been independently observed by Sir John F. W. Herschel, who 

 was the first to recognise its importance in relation both to the radia- 

 tion of heat from the moon and to the phenomena of the earth's 

 atmosphere. The reviser of the present article has constantly 

 observed, since Sir J. Herschel called attention to the subject, that 

 when the full moon, or the moon so nearly full as to appear round, 

 rises in a cloudy sky, she soon clears of cloud a circle of sky around 

 her, which becomes larger and larger, often until the entire sky is 

 freed of cloud. Referring to the induction of Arago, from a com- 

 parison of rain registered as having fallen during a long period, that 

 a slight preponderance in respect of quantity falls near the new moon 

 over that which falls near the full moon, Sir J. Herschel remarks, 

 (' Outlines of Astronomy,' 5th edit., 432 and note), " This would be a 

 natural and necessary consequence of a preponderance of a cloudless 

 sky about the full, and forms, therefore, part and parcel of the same 

 meteorological fact." [HAIL ; METEOROLOGY ; MIST ; SNOW.] 



RAIN-GAUGE, a vessel for measuring the quantity of rain which 

 f.ilN ^n any particular part of the earth's surface, the^quantity being 

 indicated by the depth of the precipitated water which would cover 

 the ground about the spot, supposing the ground to be horizontal and 

 that the water could neither flow off nor penetrate into the soil. 



In order to ascertain the quantity of ram which has fallen during 

 the continuance of a shower, it might suffice to place a prismatical or 

 cylindrical vessel, open at the top, in a horizontal position on the 

 ground or on the top of a building, and, when the shower has ceased, 

 to measure the depth of the water in the vessel by a scale of inches. 

 But, unless the depth were ascertained immediately, a portion of the 

 water would be carried off from such a vessel by evaporation, and the 

 measure would be less than it ought to be. The difficulty also of 

 ascertaining the true amount of a small depth of water would render 

 the instrument of no practical use. For the purpose therefore of 

 obtaining a more correct estimate of the quantity of rain, it has always 

 been the practice to receive the water in a second vessel, or in a tube, 

 the area of whose horizontal section is less than that of the first, so 

 that the height of the column may be greater. And, since the heights 

 of equal quantities of water in two prismatical or cylindrical vessels 

 are inversely proportional to then- bases, it is easy to perceive how a 

 rod may be graduated so as to show, in inches, the depth of water in 

 the upper vessel, and consequently the depth which would have lain 

 on the ground if no absorption had taken place. 



Originally this instrument, which has been called indifferently 

 udometer (v$ap and n&rpor), pluviometer (pluvia), and ombrometer 

 (on&pot, rain), was nothing more than a prismatical box, having a 

 square base, open at the top and communicating with a prismatical 

 box, placed vertically under it, by means of a pipe open at both ends ; 

 the area of a horizontal section of the lower box being, for the reason 

 above given, less than that of the upper box. But it is evident that a 

 prismatical or cylindrical vessel must retain, by adhesion to its sides 

 and bottom, a sensible portion of the water which enters it; and 

 inently the depth measured in such vessel must indicate a quan- 

 tity of rain less than that which has really fallen; it has therefore 



ARTS AX1> StI. 1HV. VOL. VI. 



been customary of late to make the upper part of the vessel in the 

 form of a funnel, or inverted cone. 



The most general construction of a rain-gauge is shown in the sub- 

 joined diagram, which represents a vertical section of the instrument. 

 The part CDE is a conical funnel, open both at 

 top and bottom, and the lower extremity enters 

 into the cylinder F G below, which thus receives 

 the rain from the funnel. The rod AB passes 

 through a perforation in a bar c D (in the direction 

 of a dianjL^.er of the cone at its upper surface), 

 and is attached, at B, to a circular piston, which 

 has nearly the same diameter as the interior of 

 the cylinder : the weight of the piston and rod is 

 such as to allow the former to float with its upper 

 surface on a level with the surface of the water ; 

 and the graduations, which are numbered towards 

 B, commence from a point a on a level with the 

 upper surface of the bar c D, when the piston B 

 touches the bottom of the cylinder. A rim, of a 

 cylindrical form, rises a little way above the upper 

 extremity of the conical part of the funnel, in 

 order to prevent the ram-water, which would 

 strike the interior of the latter near that extremity, 

 from being thrown out in consequence of the 

 shock. 



The diameter of the funnel at the top may be 

 12 niches, and that of the cylinder 6 inches; in 

 which case the area of the horizontal section on 

 which the rain falls will be to that of the cylinder 

 in the ratio of four to one. Hence a depth of water equal to one inch 

 at the horizontal section will be expressed by a space equal to four 

 inches on the length of the rod; and, each of such spaces being 

 divided into 100 parts, the depth of water at the said section will b* 

 indicated in hundredths of an inch. The height of the cylindrical 

 vessel below the funnel may be from 25 to 30 inches. 



For the sake of diminishing the evaporation and of measuring small 

 quantities of rain with greater precision, the diameter of the cylinder 

 is sometimes reduced to two inches, and the collected water is, by 

 means of a small pipe, inserted in the bottom of the cylinder, and 

 furnished with a cock made to pass into a glass tube whose interior 

 diameter is half an inch. In this case, the diameter of the upper 

 extremity of the funnel being the same as before, the area of the 

 surface which receives the rain from the atmosphere will be to the area 

 of a horizontal section of the glass tube as 576 to 1. Consequently a 

 shower of rain whose depth on the ground might be one-hundredth 

 part of an inch, would be indicated by 5'76 inches in the tube. 



The funnel of the cylinder may be of tin or copper, and, however 

 the instrument be constructed, it is evident that it should be placed in 

 a vertical position in some place where no object may interfere with 

 the free descent of the rain into the funnel. For the reasons stated in 

 the previous article, the gauge should be placed as near the ground as 

 possible, but the height should always be stated. At Greenwich there 

 are several gauges, at different heights, and these record very different 

 quantities of rain : for example, in 1846, the gauge 50 feet above the 

 ground recorded 13'46 inches of rain ; that at 24 feet, 22'63 inches ; 

 that at 2 feet, 25'86 inches. It is usual to observe the quantity of 

 water in the vessel every morning, if rain has fallen during the pre- 

 ceding twenty-four hours ; but, as some evaporation will take place, it 

 would be advantageous to make the observations more frequently. 



The sum of all the depths observed during any period of time, as a 

 day, month, or year, will give the whole quantity of rain which has 

 fallen in that time at the place. It is supposed that the rain falls 

 uniformly over the tract of ground lying within the limits of the 

 shower, and consequently that the quantity which passes through the 

 circular area at the upper surface of the cone is equal to that which 

 falls upon an equal area of ground anywhere within those limits. 

 During windy weather rain may fall obliquely, so that a portion of it 

 may not enter the gauge ; hence, gauges have been formed with 

 openings inclined to each of the four principal quarters, and these are 

 said to register more rain than ordinary gauges. The self-registering 

 rain-gauge is described under ANEMOMETER. 



A rain-gauge is a very imperfect instrument, and can never serve 

 further than to give an approximation to the quantity of rain which 

 may have fallen, since some of the water will always adhere to the 

 sides of the vessel, but the following method of ascertaining the allow- 

 ance to be made for the quantity thus lost has been recommended : 

 Let a sponge be made damp, yet so that no water can be squeezed from 

 it, and with this collect all the water which adheres to the funnel and 

 cylinder after as much as possible has been drawn off; then, if the 

 sponge be squeezed, and the water from it be received in a vessel 

 which admits of measuring its quantity, a near estimate may be made 

 of the depth due to it ; and this being added to the depth given by 

 the instrument would probably show very correctly the required depth 

 of rain. 



The simplest form of rain-gauge is perhaps that by Professor 

 Fleming. It consists of a cup or receiver, with the opening flush with 

 the surface of the soil. The water is delivered by a funnel into a 

 graduated jar below, which can be from time to time inspected. A, 



3o 



