338 



NATURE 



[January 31, 190 1 



the latter, by its buoyancy, will rise much higher than if there 

 were no cumulus cloud ; it may pass upward into the so-called 

 hail region, where water drops and ice particles may coexist, 

 and still higher up into the region where only ice and snow can 

 exist. 



(4) Raindrops falling from relatively warm clouds through a 

 very cold stratum of air below may be frozen into sleet before 

 they reach the ground. 



To these four elementary methods of forming atmospheric ice 

 we have to add the mechanical processes by which the small 

 particles accumulate as large hailstones. Undoubtedly much 

 light was thrown upon this subject by the notes made by ob- 

 servers on Pikes Peak during the early years of the occupation 

 of that station. 



In the thermodynamic studies of Hertz and von Bezold is 

 employed the expression " the hail stage," viz. that stage in 

 which the temperature of 32° prevails in an ascending mass 

 of moist air. It is supposed that the ascending air, having 

 already cooled to the dew-point, is carrying up with it a 

 quantity of water, either in small cloud particles or in large 

 raindrops. When these have ascended to the level where the 

 rising mpist air is cooled to the temperature of freezing, they 

 continue to give up to the air a little of their specific heat 

 until they are themselves frozen into hail or sleet. There is, 

 therefore, a thin layer of air in which this process of freezing 

 is going on and where the rising mass of mixed air and rain 

 is kept at a uniform temperature until all the water is con- 

 verted into ice. This is spoken of by Hertz as the hail stage ; 

 below it is the rain stage and above it is the snow stage. In 

 this latter region the ascending air, being already cooled be- 

 low the freezing point, can deposit its moisture only as snow 

 or small crystals of ice. Now the actual hailstones observed 

 on Pikes Peak are so frequently composed of snow that has 

 been partly melted and refrozen, or mixed with water drops 

 and refrozen, that we cannot suppose them to have been 

 wholly formed within the thin layer known as the Hertzian 

 hail stage. It is more likely that they are formed partly 

 within that and partly within the Hertzian snow stage. The 

 memoir of Hertz assumes throughout that the changes of tem- 

 perature within the ascending air are strictly adiabatic. This 

 requires that the ascent be so slow that the drops of water carried 

 upward maintain the same temperature as the surrounding air. 

 But these two conditions are almost physically incompatible ; it is 

 probable that neither of them are ever realised in nature. Among 

 other combinations that are possible and may help to explain the 

 great variety of forms of hailstones that are caught upon the 

 summit of Pikes Peak, we may suggest the following as the 

 most common : — 



(i) Frozen raindrops carried very rapidly upward through the 

 Hertzian hail stage may continue on into the snow stage and 

 grow by the accretion of snowflakes until they are finally dropped 

 to the earth, in which latter process they continue increasing 

 their snowy covering. If, however, they pass through the hail 

 stage before they reach the ground in their fall, they will be 

 found to consist of an icy nucleus surrounded by a snowy envelope 

 and covered over all by a layer of a frozen mixture of ice and 

 snow. 



(2) Air that has ascended into the snowy stage without going 

 through the rain or hail stage, or, at least, to a very slight extent", 

 because of its dryness, may form large snowballs high above the 

 Peak before beginning to fall. As such balls descend very 

 rapidly, the interior retains a low temperature, while the 

 exterior is slightly warmed and melted by the action of the 

 warmer air that the snowballs find near the ground. The result 

 is large hailstones, consisting each of a thin layer or crust of ice 

 and a snowy mass within. 



(3) In the formation of snow and hail in the midst of as- 

 cending currents of air, we must expect to notice the same 

 phenomenon as in the formation of rain, viz. after the first 

 condensations have taken place upon dust and foreign sub- 

 stances the rising mass of cloud represents dustless air in the 

 presence of water particles, but cooled by expansion to such 

 an extent that the air between the drops, or the ice spicule, 

 is in a state of supersaturation. When this condition has 

 become too intense, large quantities of aqueous vapour sud- 

 denly condense, rushing together into large drops of rain or 

 large masses of snow, and carrying with them all the finer 

 particles within their respective spheres. At the very low 

 temperatures at which this occurs, water will hold consider- 

 able air in solution, and additional air is also included at the 



NO. 1 63 1, VOL. 63] 



centre of the snowball among the particles of snow and ice. 

 Such large snowballs are heavy enough to descend rapidly 

 from the snowy stage, through the rain and hail stages to the 

 ground, and in so doing they become saturated with water 

 which recrystallises forming solid hailstones, but at the centre 

 of the mass they still hold, confined, the air originally included 

 in the snowball, and this is compressed under several atmo- 

 spheres, as was shown in 1869, by P. Reinsch (see Pogg. Ann., 

 1871, or Phil. Mag., 1871, vol. xlii. p. 79), who observed 

 that when such hailstones are melted under water the little 

 bubble of air at the centre is seen to suddenly escape and ex- 

 pand sufficiently to demonstrate the existence of a pressure of 

 fifty atmospheres under which it was confined. In this formation 

 of snowballs and the resulting hail from supersaturated air 

 within the snow stage there is an electric disturbance entirely 

 analogous to that which takes place when great drops of rain 

 are formed within the rain stage. In both cases violent thunder 

 and lightning are observed just before the fall of the hail or the 

 rain. 



These and other hypotheses that might be framed relative to 

 the methods of formation of the various kinds of hailstones 

 must, however, only be regarded as suggestions intended to 

 stimulate experimental and theoretical research in this direction. 

 One cannot doubt but that the history of the formation of hail 

 is written in its structure if we could but interpret it, 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE. 



Oxford. — Prof. W. J. SoUas has been elected to a fellow- 

 ship at University College. This fellowship, however, is not to 

 be regarded as attached to the professorship of geology ; the 

 election concerns the present professor only, the College being 

 under no obligation to his successor. 



Prof. Townsend, the new Wykeham professor of physics, has 

 come into residence, but the space intended for his laboratory 

 will not be available until the Radcliffe Library has been trans- 

 ferred to its new building. 



Cambridge. — The Clerk- Maxwell studentship in physics, 

 tenable at the Cavendish Laboratory, has been awarded to Mr. 

 H. A. Wilson, Trinity. Mr. P. V. Bevan has been appointed 

 assistant demonstrator in physics, in succession to Prof. Town- 

 send. Mr. J. C. M. Garnett has gained the Sheepshanks 

 astronomical exhibition at Trinity College. Mr. L. Whibley, 

 Fellow of Pembroke, has been appointed assistant to the 

 secretary of the University Press Syndicate. Mr. Yule Oldham, 

 Reader in geography, is lecturing this term on the hydrosphere, 

 and on the geography of Central Europe. A grant of 50/. for 

 the current year has been made to the Department of Pathology, 

 towards the course of instruction in bacteriology for the diploma 

 in public health. 



The Report of the U.S. Commission of Education for the 

 year 1898-99 has been received. It is a volume of thirteen 

 hundred pages, containing papers and statistics on many 

 branches of educational activity in various countries. Among 

 the subjects of papers of interest in connection with instruction 

 in sciences are school gardens, by Herr E. Gang ; the teaching 

 of geography, by Dr. A. J. Herbertson and others ; manual 

 training in Germany ; minor mental abnormalities in children 

 as occasioned by certain erroneous school methods ; and an 

 annotated chronological list of American text-books on arith- 

 metic, prepared by Drs. J- M. Greenwood and A. Martin. 



SCIENTIFIC SERIALS. 

 Symonis Monthly Meteorological Magazine, January. — 

 Heavy rainfall of December 30, 1900. The official weather 

 charts showed that the centre of a barometric depression lay 

 over the middle of Ireland in the morning of that day, and that 

 it moved southeastward^, passing Bristol in the early evening, 

 and reaching the English Channel on the morning of the 31st. 

 This storm was remarkable for the heavy rains which fell in the 

 valley of the Severn and its tributaries. Falls exceeding two 

 inches in 24 hours occurred over a broad diagonal belt from the 

 mouth of the Severn to the mouth of the Humber, while amounts 

 exceeding three inches occurred in a narrow strip running for 

 about 85 miles in a northeast direction from near Bristol and 

 Chepstow, covering an area of nearly 1000 miles.— The mild 

 December. The mean temperature for the month in the north- 



