ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 225 



X3articles are attracted with ever-increasing strength. In this way, in 

 addition to mere impact, in the course of a fall of 10,000 or 15,000 feet, 1 

 are formed those large hailstones which devastate crops and kill animals- 

 Taking Aitken's observations of the number of particles of water or 

 droplets of fog, falling upon a square inch in a minute in a dense fog, 

 as a criterion — say, namely, an average of about 10,000 droplets — and 

 assuming that these drops fall at the rate of not more (it is probably 

 much less) than 10 feet a minute, a hailstone falling through 10,000 feet 

 of dense cloud would encounter if it began as a snowflake, 1 inch 

 square, about 10,000,000 droplets, by mere impact. Some hailstones 

 may result from the attraction of small spicules of ice and particles of 

 water alternately as the nucleus passes through different strata, aud 

 these show concentric bands alternately opaque and clear. Similar 

 bands may be formed by the passage of the hailstone through alternate 

 spaces of thick cloud and of clear, unclouded, but saturated air. The 

 latent heat brought into the sensible condition by condensation and 

 congelation has been supposed to make such an accumulation in clear, 

 saturated air impossible, but actual observation indicates that the rapid 

 passage of the hailstone through very cold air speedily aud continuously 

 dissipates the heat thus set free. The appearance of spaces between 

 successive tiers of dense cumulus cloud and the almost invariably 

 excessive display of electric phenomena are characteristic of great 

 hailstorms. It is very probable that between the dense clouds lie 

 masses of saturated, or even supersaturated, almost dust-free air. A 

 cold hailstone falling through these would accumulate ice in clear, 

 alternate zones surrounding the nucleus. Large hailstones are gen- 

 erally spheroidal, small ones conical, with icy bases and a softer apex. 

 The large hailstones are probably more dependent on electric attrac- 

 tion, and the small on the impact of descent, for their form aud icy 

 accumulations. 



In a thunderstorm or shower, the lower clouds are generally nega- 

 tively and the upper positively electrified. Before a hailstorm clouds 

 of great significance may be observed, which may be described as tur- 

 reted cumulus or cumulo- stratus. They are quite distinctive of hail- 

 storm weather, though of course the hailstorm may not occur iu the 

 district where they are seen. They consist of hard-looking, sharply 

 defined, generally white, and rather small masses of cloud, with pro- 

 jections towering upward and rather broader at the top than at the 

 base, or equally broad. These peculiar clouds are worthy of note with 

 the view of forecasting the probable occurrence of hailstorms. 



Yapor, when it ceases to exist as a gas in the air, assumes several 



'The height of cumulus cloud may often he well ohserved and measured not 

 only from the plain, hut on mountains. The tower of cumulus cloud often exceeds 

 10,000 or 15,000 feet, and in great storms may he 25,000 to 40,000 feet from hase to 

 summit. Both observations from the earth and halloon ascents supply evidence to 

 this effect. 



sm 95 15 



