These figures were deduced by Binnie (_Min. Proc. Inst. C.E._, vol. cix.) from an examination of rainfall figures obtained over long periods of time at many places scattered over the world. The errors may, of course, be plus or minus. They are the averages of the errors actually found, and are themselves subject to fluctuations. Thus the 15 per cent. error for a five-year period may be 16 or 13, the 8 per cent. error for a ten-year period may be 8½ or 7½, with similar but minute fluctations for the other periods. Binnie’s figures also show that the ratio of the fall at any place in the driest year to the mean annual fall, averages ·51 to ·68, with a general average of ·60, and that the ratio in the wettest year to the mean annual fall averages 1·41 to 1·75, with a general average of 1·51. For India the general averages are ·50 and 1·75. These figures are useful as a means of estimating the probable greatest and least annual fall, but they are averages for groups of places. The greatest fall at any particular place may occasionally be twice the mean annual fall. At some places in India, in Mauritius, and at Marseilles it has been two and a half times the mean annual fall. The least annual fall may, in India, be as low as ·27 of the mean. In England the fall in a dry year has, once at least, been found to be only ·30 of the mean annual fall. The mean fall (average for all places) in the three driest years is, from Binnie’s figures, about ·76 of the mean annual fall. The figures given above, except when a particular country is mentioned, apply to all countries and to places where the rainfall is heavy, as well as to those where it is light. But in extremely dry places the fluctuations are likely to be much greater. At Kurrachee, with a mean annual fall of only 7·5 inches, the fall in a very wet year has been found to be 3·73 times, and in a very dry year only ·07 times the mean annual fall. In the United Kingdom the probable rainfall at any place in the driest year may be taken as ·63 of the mean annual fall. For periods of two, three, four, five and six consecutive dry years, the figures are ·72, ·77, ·80, ·82, and ·835. These figures are of importance in calculations for the capacity of reservoirs (CHAP. XIII., _Art. 2_).