Page:Popular Science Monthly Volume 10.djvu/544

526 the air (which he hoped would moisten the powdered plaster sufficiently to cause it to set firmly into whatever form it collected). The jet was directed against a splinter of wood.

In this way masses of plaster very closely resembling hailstones were obtained. They were all more or less conical, with their bases facing the jet. But, as might be expected, the angles of the cones were all smaller than those of the hailstones. Two of these figures are shown in the sketches annexed.

The striæ were strongly marked, and exactly resembled those of the hailstone. The bases also were rounded. They were somewhat steeper than those of the hailstone; but this was clearly due to the want of sufficient cohesive power on the part of the plaster. It was not sufficiently wet. Owing to this cause also it was not possible to preserve the lumps when they were formed, as the least shake caused them to tumble in pieces.

Similar masses were also obtained by blowing the vapor of naphthaline, but these were also very fragile. Whereupon it is remarked: At ordinary temperatures the powdered naphthaline does not adhere like ice when pressed into a lump. No doubt at very low temperatures ice would behave in the same way, that is to say, the particles would not adhere from the force of impact. Hence it would seem probable that, for hailstones to be formed, the temperature of the cloud must not be much below the freezing-point.

That the effect of the temperature of the cloud exercises great influence on the character of the hailstones cannot be doubted. And if, as has been suggested by M. L. Dufour, the particles will sometimes remain fluid, even when the temperature is as low as 0° Fahr., it is clear that, as they are swept up by a falling stone, they may freeze into homogeneous ice either in a laminated or crystalline form.

The author then proceeds to show that raindrops are probably formed in the same way as hailstones; that, although the raindrops have no structural peculiarities like the hailstones, the aggregation of the particles of water by the descent of the drop through the cloud is the only explanation which will account for them. He shows that, as Mr. Baxendell had previously pointed out, the amount of vapor which a cold drop could condense before it becomes as warm as the vapor would be inappreciable when compared with the size of the drop; and since, in order that there might be condensation, the air must be warmer than the drop, the drop could not part with its heat to the air. He also shows that during the time of descent of a large drop the heat lost by radiation would not account for the condensation of sufficient vapor to make any appreciable difference in the size of the drop. Whereas, if we suppose all the vapor which a body of saturated air at 60° Fahr. would contain over and above what it would contain at 32° to be changed into a fog or cloud, then if a particle, after commencing to descend, aggregated to itself all the water suspended