Genesis 7:24

And the waters prevailed upon the earth a hundred and fifty days - The breaking up of the fountains of the great deep, and the raining forty days and nights, had raised the waters fifteen cubits above the highest mountains; after which forty days it appears to have continued at this height for one hundred and fifty days more. "So," says Dr. Lightfoot, "these two sums are to be reckoned distinct, and not the forty days included in the one hundred and fifty; so that when the one hundred and fifty days were ended, there were six months and ten days of the flood past." For an improvement of this awful judgment, see the conclusion of the following chapter, Genesis 8:22 (note).

Fifty and a hundred days. - These, and the forty days of rain, make one hundred and ninety days: about six lunar months and thirteen days. If to this we add the month and seventeen days before the commencement of the rain, we have eight months completed, and are therefore brought to the first day of the ninth month. The waters may be said to prevail as long as the ark had its full draught of water. It is probable they were still rising during the first half of the hundred and fifty days, and then gradually sinking during the other half.