Page:Popular Science Monthly Volume 10.djvu/542

524 overtake them in their descent; and then the smaller stones will stick to the larger and at once deform them. But besides the deformation caused by the presence of the smaller stone, the effect of the impact may be to impart a rotary motion to the stone, so that now it will no longer continue to grow in the same manner as before. Hence we have causes for almost any irregularities of form in the ordinary hail-stone.



It appears, from the numerous accounts which have been published, that occasionally hailstones are found whose form is altogether different from that described above. These, however, are exceptional, and, to whatever causes they may owe their peculiarities, these causes cannot affect the stones to which reference is here made.

Again, on careful examination, it is seen that the ordinary hail-stones are denser and firmer toward their bases or spherical sides than near the vertex of the cone, which latter often appears to have broken off in the descent. This, also, is exactly what would result from the manner of formation described above. When the particle first starts it will be moving slowly, and the force with which the particles impinge upon it will be slight, and consequently its texture loose; as, however, it grows in size and its velocity increases, it will strike the particles it overtakes with greater force and so drive them into a more compact mass. If the velocity were sufficient, the particles would strike with sufficient force to adhere as solid ice, and this appears to be the case when the stones become large, as large as a walnut, for instance.

An idea of the effect of the suspended particles, on being overtaken