164 



NATURE 



{Dec. 21, 1876 



on the density and thickness of the cloud below the point 

 from which the drops started. 



The author's object is to suggest that this is the actual 

 way m which raindrops and hailstones are formed. 

 He was first led to this conclusion from observing 

 closely the structure of ordinary hailstones. Although 

 to the casual observer hailstones may appear to have 

 no particular shape except that of more or less im- 

 perfect spheres, on closer inspection they are seen ail 

 to partake more or less of a conical form with a rounded 

 base like the sector of a sphere. In texture they have 

 the appearance of an aggregation of minute particles of 

 ice fitting closely together, but without any crystalUsation 

 such as that seen in the snow-fJake, although the surface 

 of the cone is striated, the strise radiating from the vertex. 

 Such a form and texture as this is exactly what would 

 result if the stones were formed in the manner described 

 above. When a particle which ultimately formed the 

 vertex of the cone, started on its downward descent and 

 encountered other particles on its lower face, they would 

 adhere to it, however slightly. The mass, therefore, would 

 grow in thickness downwards ; and as some of the par- 

 ticles would strike the face so close to the edge that 

 they would overhang, the lower face would continually 

 g'ow broader, and a conical form be given to the mass 

 above. 



When found on the ground the hailstones are gene- 

 rally imperfect ; and besides such bruises as may be 

 accounted for by the fall, many of them appear to 

 have been imperfect before reaching the ground. Such 

 deformities, however, may be easily accounted for. 



The larger (stones fall faster than those which are 

 smaller, and consequently may overtake them in their 



Fig. 3. — Imitation in Plaster of Paris. 



descent ; and then the smaller stones will stick to th« 

 larger and at once deform them. But besides the defor- 



FiG 4. — Imitation in Plaster of Paris. 



mation 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 hailstones are denser and firmer towards 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 t 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 by the stone, may be formed from the action 

 of the particles of sand in Mr. Tilghman's sand-blast, used 

 for cutting glass. The two cases are essentially the same, 

 the only difference being that the hailstone is moving 

 through the air, whereas in the case of the sand-blast, the 

 object which corresponds to the stone is fixed, and the 

 sand is blown against it. 



By this sand-blast the finest particles of sand are made 

 to indent the hardest material, such as quartz or hard 

 steel ; so that the actual intensity of the pressure between 

 the surface of the particles of sand and that of the object 

 they strike, must be enormous. And yet the velocity of 

 the blast is not so much greater than that at which a good- 

 sized hailstone descends. It is easy to conceive, there- 

 fore, that the force of the impact of the suspended par- 

 ticles of ice if not much below the temperature of freezing 

 on a large hailstone, would drive them together so as to 

 form solid ice. For the effect of squeezing two particles 

 of ice together, is to cause them to thaw at the surface of 

 contact, and as soon as the pressure is relieved they freeze 

 again, and hence their adhesion. 



It is then shown that hailstones, such as those described, 

 can neither be formed by the freezing of rain-drops, nor by 



