The process of rain formation is not well understood. As we have seen, the existence of nuclei in the air serves to explain why, when the conditions of temperature and humidity are right, moisture condenses in the tiny droplets that constitute clouds. The difficulty is to explain how, at certain times, quantities of drops are formed of a size large enough to carry them rapidly to the earth. The number of nuclei is so great that, as Humphreys has pointed out, even if all the water vapor in a volume of humid air was condensed upon them, the size of each drop would remain very small. He has suggested that, in a column of rising air, the small drops formed at the base of a cloud filter out most of the nuclei, so that at greater heights there are relatively few of the latter, which can, therefore, gather sufficient water about them to form drops of “falling” size. The speed with which drops fall through the air, which is only a fraction of an inch per second for the average cloud droplet, increases rapidly with the size of the drop up to a certain point, but for the drops that reach the earth as rain the speed of fall tends to become approximately uniform. Several investigators have measured the size of raindrops. One method of doing this is to allow the drops to fall into a shallow layer of fine, uncompacted flour. Each drop forms a little pellet of dough, which is found, by experiments with previously measured drops (produced for the purpose and dropped from various heights), to correspond very closely with the size of the drop. These pellets dry and harden, and can then be carefully measured, photographed, etc. Hundreds of samples of raindrops were thus measured by Mr. W. A. Bentley of Jericho, Vt., and the measurements were tabulated with reference to the kinds of clouds from which they fell, the distribution of large and small drops in the different parts of a storm, and other circumstances attending their fall. Drops of very different dimensions are found to fall at one time. The commonest sizes recorded by Bentley were from one-thirtieth to one-eighth of an inch in diameter; but many drops too minute to form casts were estimated to be less than a hundredth of an inch in diameter, while the largest drops observed had a diameter of a quarter and even a third of an inch. This range of size corresponds to a range in the rate of fall from about five feet a second for the smallest drops up to about twenty-five feet a second for the largest. The maximum size of raindrops is limited by the fact that very large drops are broken up in their fall through the air. Theoretically, the limiting size is somewhat less than the largest sizes found by Bentley. [Illustration: CASTS OF RAINDROPS FROM A THUNDERSHOWER _Collected and Photographed by W. A. Bentley_]