DISCOVKRY 



21 



WTien it is said that an inch of rain has fallen, it is 

 meant that sufficient water has fallen to fill to over- 

 fiovving a vessel i inch long, i inch wide, and i inch 

 deep) — that is, a cubic inch of water has fallen on 

 every square inch of land. Since an acre contains 

 6,272,640 square inches, an inch of rain on this area 

 means that loi tons of water have fallen. For the 

 amount fallen is 6,272,640 cubic inches ; 1728 cubic 

 inches equal one cubic foot ; a cubic foot of water 

 weighs 62 J lb. ; and so, by calculation, we arrive at 

 the above result. 



Having decided the kind of gauge to use, our next 

 problem is to select a site as free as possible from such 

 obstructions as buildings, trees, and walls. A good 

 plan is to place the instrument as far away from the 

 obstacle as the obstacle is high. The late Dr. R. H. 

 Scott, formerly Secretary and Director of the Meteoro- 

 logical Office, recommended that " the angle subtended 

 in each azimuth by the nearest obstacle . . . should 

 not exceed 30 degrees, and the true bearing of the 

 obstacle from the gauge should be carefully measured 

 and noted in the register." In plain language, the 

 Meteorological Office recommends that the gauge be 

 twice the distance from the obstacle as the height 

 of the object in question, though it must be admitted 

 that this is not always possible in small gardens. In 

 a desire to avoid obstruction, we must avoid the 

 other extreme. The site must not be too exposed, 

 nor should it be on sloping ground. 



The rain-gauge is best fixed into the ground by 

 fitting its base accurately into a hole lined with cement- 

 concrete. Meteorological Office gauges are frequently 

 made with splayed bases, and if these are firmly 

 embedded in cement, it is quite impossible for them 

 to blow over. 



All gauges should have a capacity of 4 inches 'at 

 least, for it is possible that rain sufficient to fill a gauge 

 of this size may fall in the British Isles in 24 hours. 

 Indeed, falls of as much as 9 and 10 inches have been 

 known to take place in this country. 



We will now suppose that a fall of rain has taken 

 place. We visit the gauge, which, we will assume, 

 is marked in inches, at 9 a.m. G.M.T.,' the usual 

 hour for making such observations. We remove the 

 funnel, laying it carefully aside, take out the inner 

 can or bottle, as the case may be, and pour the contents 

 into the Camden measure-glass. Each mark on the 

 glass, except the bottom one, indicates one hundredth 

 of an inch. The bottom one indicates half a hundredth. 

 When small amounts of rain have fallen, this bottom 

 mark is of importance, because if the water stands 



' Greenwich Mean Time. In official observations, time is 

 reckoned from midnight in 24-hour periods ; 9 a.m. being 

 9 hours, and 9 p.m. 21 hours, and so on. " Summer time " is 

 ignored. 



above it, the amount is recorded as o-oi inch and the 

 day is known as a " rain day." If the water does not 

 reach the mark, the record becomes " trace." 



We will assume, however, that the fall has been a 

 heavy one, and that the water has reached the twenty- 

 seventh mark. This is recorded as 0-27 (each mark 

 being a hundredth of an inch). If we find there is 

 more water in the gauge than fills the measure-glass 

 (capacity 0-50 inch), we proceed to fill it accurately 

 as many times as it is possible, and measure the 

 fraction over also. Thus, if there is sufficient water 

 to fill the measure-glass three times and to fill it for 

 the fourth time to the mark 0-23, the recorded fall 

 will be 3 X 0-50 -f- 0-23, i.e. 1-73 inches. A similar 

 procedure with a measure-glass graduated in milli- 

 metres and tenths enables a record to be made in that 

 case also. 



When snow falls instead of rain, the method of 

 measurement is modified slightly in one of two ways. 

 The first method is to remove the funnel of the gauge, 

 taking care not to lose anv of the snow retained in it. 



Fig. 4. — LIV'IN'GSTONE PORTABLE R.\IN-GAUGI3. 



and to take it and the bottle or can near to a fire, so 

 that the whole of the snow may be melted. The 

 water is then measured in the usual way. The second 

 way is to measure say i inch of warm water in 

 the measure-glass, and pour it over the snow in the 

 funnel so that it melts and runs down the tube into 

 the bottle below. The actual rainfall is then the 

 whole amount measured less the amount added to 

 melt the snow. Hail may be dealt with in the same 

 way. 



When positions are chosen for rain-gauges where 

 they cannot be visited daily, it is necessary to employ 

 a mountain rain-gauge such as the Bradford. The 

 Bradford is a Snowdon gauge modified to suit the 

 purpose for which it is intended. Its funnel is 5 inches 

 in diameter, but the inner can is made to contain 

 from 12 to 30 inches according to the rainfall of the 

 district in which the gauge is to be used. This gauge 

 measures, of course, the monthly rainfall, if the readings 

 are made once a month (usually at 9 a.m. on the first 

 of the month). Before measuring the quantity of 

 water in the usual way with the measure-glass, the 



