NA TURE 



[July 28, 1904 



for flowing water already in use. In tlie United 

 States, as in tliis country, the cubic foot is 

 taken as the unit of volume and the second as 

 the unit of time when measuring flowing water in 

 streams, while here the gallon is generally adopted 

 as the unit when dealing- with supplies for domestic 

 purposes. In the United States the million gallons 

 in twenty-four hours appears to be recognised as a 

 standard for city water supply, and an acre in area 

 covered one inch or a foot deep in a month or a vear 

 is used for irrigation purposes. The unit of inches of 

 rainfall per acre on the catchment area and the result- 

 ing run off in gallons for town supplies, or in cubic 

 feet for drainage, is a measure of very general adop- 

 tion. In India many irrigation engineers have adopted 

 the term " cusecs " as representing cubic feet per 

 second. 



With regard to the proportion of rainfall that finds 

 its way into a stream, the author deprecates the use 

 of averages, and expresses the opinion that safe de- 

 ductions can only be obtained from using the minimum 

 rainfall and taking into account the longest period 

 such minimums may be expected to occupy. The 

 records of the United States show that this minimum 

 period may be expected frequently to last more than 

 three years. 



In this country the general rule is to take the 

 average of the longest period over which the rainfall 

 records of the district extend, from this to deduct one- 

 fifth to allow for the mean annual rainfall of the three 

 consecutive driest years, and from the product further 

 to deduct from eleven to fifteen inches for loss by 

 evaporation, soakage, &c., according to the character 

 of the ground, the remainder giving the quantity 

 available for storing. If compensation water has to 

 be provided, a further deduction of one-third of the 

 available suppl)' has to be made. Fourteen inches is 

 commonly taken as the figure representing evapor- 

 ation, &c. , in this country. For example, with an 

 average annual rainfall of thirty inches, ten inches 

 would be available for run off or storage, or, where 

 compensation water has to be given, 6.67 inches would 

 be available for storage. Taking an inch of rainfall 

 as 3630 cubic feet per acre, 10 inches would give 36,300 

 cubic feet or 226,300 gallons to the acre of gathering 

 ground. 



.\s a general statement, Mr. Rafter's investigations 

 have led him to the conclusion that the minimum 

 rainfall varies from half to one-fourth the maximum. 



The late Mr. Symons's proportion for this country 

 was that the rainfall for the wettest year was half as 

 much more than the mean, and for the driest year 

 one-third less, or, taking the average of three wettest 

 years, one-fifth less than this average. 



Mr. Rafter considers that averages derived from a 

 shorter period than thirty-five years are not to be relied 

 on within 2 per cent. The same conclusion was 

 arrived at by Mr. H. R. Binnie in his paper on the 

 average annual rainfall reported in the minutes of 

 proceedings of the Institution of Civil Engineers, 1892. 

 This figure was derived from an examination of rain- 

 fall statistics from 153 stations situated all over the 

 world. While short periods like five years' average 

 gave an error of 32 per cent., and thirty years 5-8 per 

 cent., the error for thirty-five year periods was only 

 25 per cent., and fifty years came no closer. 



.'\lthough the annual quantity of rainfall varies very 

 much in different localities and in different countries, 

 the same law universally applies as to the relation of 

 the wettest and driest years to the average fall if taken 

 over a sufficiently long period. 



.^s pointed out by the author of the report, caution 

 is necessary in taking the average of the rainfall as 

 a guide ; for storage purposes, where the water has to 



NO. 18 I 3, VOL. 70] 



be collected in a reservoir a minimum fall derived 

 fiom an average of years may be a trustworthy guide, 

 but where provision has to be made for carrying off 

 the water in artificial channels for drainage purposes, 

 or where the water has to be pumped, as in low-lying 

 districts, the data to be ascertained is the maximum 

 rainfall that has to be dealt with in a short period. 

 Thus, while the rainfall of the year, or even of the 

 winter months, may not have been excessive, yet 

 floods may have ensued due to heavy rain falling for 

 a few days on ground already saturated. In the Fen 

 districts on the east coast of England, which depend 

 entirely on artificial drainage, the rule is to allow for 

 a discharge equal to a continuous fall of a quarter 

 of an inch of rain during twenty-four hours. The 

 mean daily fall of the rain which caused the twenty- 

 one floods in the Witham district since 1852 was an 

 average of 0-26 inch spread over seventeen days ; 

 the average annual fall of the district for the wet 

 period was 32-39 inches, and over a period of seventy 

 years 22.93 inches. The greatest fall during this 

 period averaged 0-41 inch spread over fourteen days, 

 in November, 1885, and also in October, 1883, when 

 there was considerable flooding.' 



The figures given in this paper show that in the 

 eastern States of America with a maximum rainfall 

 of from 20 to 60 inches half the rainfall runs off, and 

 that with a minimum fall from a fourth to a sixth. 

 In the western States, with a fall of about 12 inches, 

 the run off varies from half an inch to an inch. 



The total run off of a stream depends very largely 

 on the run off of the storage period. Usually about 

 0-75 to 0-85 of the total rainfall of this period runs off 

 in the stream, while for the summer, or growing 

 period, not more than about o-i of the rainfall appears, 

 this small quantity being due to evaporation and 

 absorption by vegetation. The total run off for the 

 year depends very largely on whether or not the rain- 

 fall from December to May is large or small. Whether 

 any given stream is low during the summer months 

 or has then a well sustained flow will depend very 

 largely on the rainfall of the month of May. When 

 the May rainfall is heavy enough to produce full 

 ground water, the flow is likely to be well sustained. 



The extent of afforestation seems to have a consider- 

 able effect on the run off of streams, catchments with 

 dense forests showing a larger run off for the same 

 rainfall than those which are deforested. 



THE ARAPAHO SUN DANCE.- 



THE scientific value of the anthropological series 

 of the Publications of the Field Columbian 

 Museum, Chicago, has been sustained by the im- 

 portant memoir on the Arapaho sun dance by Dr. 

 G. A. Dorsey, the energetic curator of the Department 

 of .'\nthropo'logy. .'Although only very recently pub- 

 lished, the work bears the date of June, 1903, which 

 will cause superfluous trouble to bibliographers. Dr. 

 Dorsey witnessed the sun dance in 1901 and 1902, and 

 he has taken great pains to give a clear and minute 

 account of this eight-day ceremony. The description 

 is illustrated with a great wealth of illustrations, there 

 being no fewer than 13^ plates, many of which contain 

 two "figures ; it is probably safe to say that no ceremony 

 has hitherto been so amply illustrated. It is also a 

 matter of congratulation that the description is so 

 detailed, as the significance of a ceremony can only 

 be adequately realised when all the details of the events 



1 '■ The Fens of South Lincolnshire " (Simpkin, Marshall and Co., Ltd.) 

 "The Drainage of Fens and Lowlands" (Spon, Ltd.). 



- "The Arapaho Sun Dance: the Ceremony of the Offerings'-lodge. 

 By G. A. Dorsey. Field Columbian Museum, Anthropological Series. 

 Vol. iv. (Chicago, U.S.A., June, 1903.) 



