If, now, the equilibrium be disturbed by an augmented humidity of the inflowing air, two results ensue. On the one hand the precipitation is increased, and on the other, the absorbent power of the air being less, the rate of evaporation is diminished. In so dry a climate the precipitation is increased in greater ratio than the humidity, and the rate of evaporation is diminished in less ratio; while of the increased precipitation an increased percentage gathers in streams and finds its way to the lake. That reservoir, having its inflow augmented and its rate of evaporation decreased, gains in volume and grows in breadth until the evaporation from the added expanse is sufficient to restore the equilibrium. Giving attention to the fact that the lake receives a greater percentage of the total downfall than before, and to the fact that its rate of evaporation is at the same time diminished it is evident that the resultant augmentation of the lake surface is more than proportional to the augmentation of the precipitation. We are therefore warranted in assuming that an increase of humidity sufficient to account for the observed increase of 17 per cent. in the size of the lake would modify the rainfall by less than 17 per cent. The actual change of rainfall cannot be estimated with any degree of precision, but from a review of such data as are at my command I am led to the opinion that an allowance of 10 per cent. would be as likely to exceed as to fall short, while an allowance of 7 per cent. would be at the verge of possibility. The rainfall of some other portions of the continent has been recorded with such a degree of thoroughness and for such a period that a term of comparison is afforded. In his discussion of the precipitation of the United States, Mr. Schott has grouped the stations by climatic districts, and deduced the annual means for the several districts. Making use of his table on page 154 (Smithsonian Contributions, No. 222), and restricting my attention to the results derived from five or more stations, I select the following extreme cases of variation between the mean annual rainfalls of consecutive decades. District I comprises the sea coast from Maine to Virginia, and the record includes five or more stations from 1827 to 1867. From the decade 1831-’40 to the decade 1841-’50 the rainfall increased 6 per cent. District II comprises the state of New York and adjacent regions, and includes five or more stations from 1830 to 1866. From the decade 1847-’56 to the decade 1857-’66 the rainfall increased 9 per cent. District IV comprises the Ohio Valley and adjacent regions, and includes five or more stations from 1837 to 1866. From decade 1841-’50 to decade 1851-’60 the rainfall diminished 8 per cent.