But much shorter periods than twenty-four hours have to be dealt with. The following figures are given by Chamier (_Min. Proc. Inst. C.E._, vol. cxxxiv.) as applicable to New South Wales, and he considers that they are fair guides, erring on the side of safety, for other countries:-- Duration of fall in hours 1 4 12 24 Ratio of fall to maximum daily fall ¼ ½ ¾ 1 The above figures are probably safe for England. For India the case is far otherwise. The following falls have been observed there:-- +----------+-------+--------------+--------+ | Period. | Fall. |Rate per Hour.|Remarks.| +----------+-------+--------------+--------+ | |Inches.| Inches. | | |7 hours | 10 | 1·43 | | |4·5 hours | 7·7 | 1·7 | | |2 hours | 8 | 4 | | |1 hour | 5 | 5 | | |20 minutes| 1·6 | 4·8 | | |10 minutes| 1 | 6 | | +----------+-------+--------------+--------+ The falls of 1 inch in ten minutes were frequently observed near the head of the Upper Jhelum Canal, a place where the annual rainfall is not more than 30 inches (see also CHAP. XII., _Art. 1_). In some parts of the Eastern Himalayas, where 30 inches of rain has fallen in a day, it is possible that 8 inches may have fallen in an hour. In England 4 inches has fallen in an hour. The heaviest falls in short periods do not usually occur in the wettest years, and they may occur in very dry years. Nor do they always occur on a very wet day. CHAPTER III COLLECTION OF INFORMATION CONCERNING STREAMS 1. =Preliminary Remarks.=--The information which is required concerning streams depends on the character of the stream and on the nature of the work which is to be done. For the present let it be supposed that the stream is large and perennial. Other kinds of streams will be dealt with in _Arts. 6_ and _7_. In dealing with a large perennial stream it is nearly always necessary to know the approximate highest and lowest water-levels, and these can generally be ascertained by local inquiry, combined with observations of water marks; but a higher level than the highest known and a lower level than the lowest known are always liable to occur, and must to some extent be allowed for. If navigation exists or is to be arranged for, the highest and lowest levels consistent with navigation must be ascertained. The highest such level depends chiefly on the heights of bridges. A plan to a fairly large scale is also necessary in most cases.