Jan. lo, 1878] 



NA TURE 



209 



Owing to the radiation of the surfaces from a common 

 vertex and the steps which occur between the vertex and 

 the base, the angle of the conical surface of the stone is 

 greater near the vertex than near the base. Thus the 

 smaller stones appear less elongated than those which 

 are larger. 



The fact that in the sketches of actual stones, which 

 I gave in my last paper, I showed the steps as less pro- 

 nounced and the angles larger than they are in the arti- 

 ficial stones, is probably owing in some measure to my 

 having formed my ideas from the observation of favourable 

 specimens chosen from amongst those which fell. The 

 larger angles were probably also, in part, owing to the 

 smaller size of the actual hailstones, which were not much 

 more than one-fourth of an inch across. But I think that 

 it is important to notice that the somewhat imperfect way 

 in which the outside layers in the surface of the artificial 

 stones are continued, may be owing to the narrowness of 

 the jet of air which, on striking the stone, tends to diverge 

 laterally rather than to flow upwards past the sides of the 

 stone, as it would do if the jet were broader, or as the air 

 must do when the stone is falling through it. 



The rate at which stones can be formed depends on 

 the amount of water which can be introduced into the 

 spray, the larger stones taking from one to two minutes. 

 At first sight this may seem to be somewhat slow, but the 

 following estimate tends to show that the artificial are 

 probably formed quicker than the actual stones. 



The speed of the jet of air at the point at which the 

 stones are formed is nearly equal to that at which the 

 larger stones would fall through the air. This is shown 

 by the fact that if a large stone becomes accidentally 

 detached from its splinter of wood it rather falls than 

 rises, but when this happens with smaller stones they are 

 driven up by the force of the blast. 



I find that the speed of the blast varies from 150 to 200 

 feet per second, i.e., from one-and-a-half to two miles a 

 minute. The larger stones, therefore, traverse from one 

 to four miles of frozen spray. So that if we imagine a 

 cloud as dense as the spray it would have to be from one 

 to four miles thick in order that the stones might, in fall- 

 ing through it, attain the size of the artificial stones ; and 

 considering that the stones would only gradually acquire 

 a speed equal to that of the blast, the time occupied in 

 fallmg through the cloud would in all probability be very 

 considerable, at least from five to ten minutes after the 

 stone had acquired a sensible size. 



As regards the proportion which the density of spray 

 bears to that of a cloud, a comparison may be made from 

 the fact that when working in saturated air at a tempera- 

 ture of 60° or 70° F,, the condensation of vapour supplied 

 sufficient ice to form the spray ; and since it is probable 

 that the dense summer clouds, from which hail is formed, 

 result from the cooling of air from temperatures nearly, if 

 not quite, equal to this, there is probably no great differ- 

 ence in the density of the clouds and the spray. 



1 have not yet had an opportunity of examining the 

 texture of these stones under the microscope, but to 

 all appearance they consist of an aggregation of small 

 spherical particles of ice ; and it seems worthy of notice 

 that while nothing like a snow crystal ever appears to be 

 produced in the ether spray, the moment the blast is 

 stopped the end of the ether tube becomes covered with 

 ice, which often assumes the form of snow crystals. 



This appears to indicate the character of the difference 

 between those conditions which result in snow and those 

 which result in hail. 



When the cloud particles are formed at or above the 

 temperature of 32° and then freeze, owing to cooling by 

 expansion or otherwise, the particles as they freeze retain 

 their spherical form. This is what happens in the 

 spray. 



On the other hand, when saturated air at a temperature 

 below 32° is still further cooled, the deposition of the 



vapour will be upon ice, and will take the form of snow 



crystals. 



The aggregation of the snow crystals into flakes is, as 

 I pointed out in my previous paper, accounted for by 

 the larger crystals overtaking the smaller crystals in their 

 descent, and the still more rapid descent of the flakes as 

 they increase in size. 



As regards the formation of rain-drops, I have nothing 

 to add to what was contained in my last paper. The 

 same explanation obviously applies to both hail and rain, 

 and any doubt which may have been left by the less 

 direct arguments in my former paper will, I venture to 

 think, have been removed by the verification of my pre- 

 dictions in the production of artificial hailstones so closely 

 resembling in all particulars those formed by nature. 

 And, in conclusion, I would thank Dr. Crompton for the 

 suggestion of the means by which I have been able to 

 produce these stones. 



OUR ASTRONOMICAL COLUMN 



The South Polar Spot of Mars.— Prof. Asaph 

 Hall has instituted a series of measures of the position of 

 the south polar spot of Mars, with the Washington 

 refractor during the late favourable opposition of the 

 planet, having been led thereto by the great discordances 

 in the positions of the spot, as determined so far. He 

 adopts Oudeman's node and inclination of the equator of 

 Mars, which, for the epoch taken, viz., 1877, September, 

 17-0, G.M.T. give N = 47° 56', I = 39° 14', and the angle 

 of position of the south pole 162° 6', and assumes the 

 time of rotation of the planet 24h. 37m. 2273s., as found 

 by Mr. Proctor. The observations were made with a 

 power of 400, and on thirty-two nights, from August 10 to 

 October 24, during the whole of which period the spot 

 was always seen with great distinctness, and little change 

 in its appearance noted except what might be accounted 

 for by change of distance. From thirty-four equations of 

 condition treated on the method of least squares. Prof. 

 Hall finds for the angle of position of the south pole of 

 Mars at the above-mentioned epoch 166° 22', for the 

 radius of the small circle described by the spot 5° 11', and 

 for the angle of position of the spot at the epoch, with 

 respect to the rotation-axis of the planet, 311° 24'. The 

 various determinations of the south polar distance of this 

 spot are as follow : — 



Herschel, 1783 ... 8 8 



Bessel, 1830 8 6 



Madler, 1837 12 o 



Secchi, 1857 17 42 



On several of the finer nights, when the markings on the 

 edge of the spot were very distinct, it appeared as "a 

 depression in the surface of the planet." 



Prof. Newcomb's Lumar Researches.— It is under- 

 stood that if no unforeseen delay occurs in the printing, 

 Part I. of " Researches on the Motion of the Moon,'' upon 

 which Prof. Newcomb has been engaged for six years 

 past, will be ready for publication in the course of next 

 month. It is devoted to the discussion of eclipses and 

 occultations previous to 1750. An abstract appeared 

 in Silliman's Journal for November last. 



The Cordoba Observatory.— In an address delivered 

 on November 4, on the occasion of receiving from the 

 Governor of the province of Cordoba the premiums 

 awarded at the Centennial Exhibition in Philadelphia to 

 the Argentine National Observatory and to himself for 

 Lunar and Stellar Photographs, Dr. B. A. Gould gave a 

 brief outline of the successive applications of photography 

 to astronomical purposes since Mr. Bond's experiments 

 with the 15-inch refractor of Harvard Observatory in 

 1850, with more particular reference to work executed at 

 Cordoba of late in this direction. Dr. Gould expresses 



