Dec. 21, 1876J 



NATURE 



165 



the condensation of vapour on a nucleus of ice ; and that 

 it is impossible that the particles of ice can have been 

 drawn together by electrical attraction — their conical 

 shape, and the increase in their density towards their 

 thicker sides clearly showing that the particles have aggre- 

 gated from one direction, and with an increasing force as 

 the size of the stone has increased. 



The possibility of making artificial stones is thus con- 

 sidered : — If a stream of frozen fog were driven against 

 any small object, then the frozen particles shculd accumu- 

 late on the object in a mass resembling a hailstone. Not 

 seeing his way to obtain such a stream of frozen fog, the 

 author thought it might be worth while to try the effect of 

 blowing very finely powdered plaster of Paris. He there- 

 fore introduced a stream of this material into a jet of 

 steam, issuing freely into the air (which he hoped would 

 moisten the powdered plaster sufficiently to cause it to set 

 firmly into whatever form it collected). The jet was 

 directed against a splinter of wood. 



In this way masses of plaster very closely resembling 

 hailstones were obtained. They were all more or less 

 conical, with their bases facing the jet. But as might be 

 expected, the angles of the cones were all smaller than 

 ihone of the hailstones. Two of these figures are shown 

 in the sketches annexed : 



The striae were strongly marked, and exactly resembled 

 those of the hailstone. The bases also were rounded. 

 They were somewhat steeper than those of the hailstone ; 

 but this was clearly due to the want of sufficient cohesive 

 power on the part of the plaster. It was not sufficiently 

 wet. Owing to this cause also it was not possible to pre- 

 serve the lumps when they were formed, as the least shake 

 caused them to tumble in pieces. 



Similar masses were also obtained by blowing the 

 vapour of naphthaline, but these were also very fragile. 

 Whereupon it is remarked : — At ordinary temperatures 

 the powdered naphthaline does not adhere like ice when 

 pressed into a lump. No doubt at very low temperatures 

 ice would behave in the same way, that is to say, the 

 particles would not adhere from the force of impact. 

 Hence it would seem probable that for hailstones to be 

 formed, the temperature of the cloud must not be much 

 below freezing-point. 



That the effect of the temperature of the cloud exer- 

 cises great influence on the character of the hailstones 

 cannot be doubted. And if, as has been suggested by 

 M. L. Dufour, the particles will sometimes remain fluid, 

 even when the temperature is as low as 0° F., it is clear 

 that as they are swept up by a falling stone, they may 

 freeze into homogeneous ice cither in a laminated or 

 crystalline form. 



The author then proceeds to show that raindrops are 

 probably formed in the same way as hailstones ; that 

 although the raindrops have no structural peculiarities 

 like the hailstones, the aggregation of the particles of 

 water by the descent of the drop through the cloud is the 

 only explanation which will account for them. He shows 

 that, as Mr. Baxendell had previously pointed out, the 

 amount of vapour which a cold drop could condense 

 before it becomes as warm as the vapour rvould be in- 

 appreciable when compared with the size of the drop, 

 and since, in order that there might be condensation, the 

 air must be warmer than the drop, the drop could not 

 part with its heat to the air. He also shows that during 

 the time of descent of a large drop, the heat lost by 

 radiation would not account for the condensation of suf- 

 ficient vapour to make any appreciable difference in the 

 size of the drop. Whereas if we suppose all the vapour 

 which a body of saturated air at 60" F. would contain 

 over and above what it would contain at 32° to be changed 

 into a fog or cloud ; then if a particle, after commencing 

 to descend, aggregated to itself all the water suspended in 

 the volume of air through which it swept, the diameter of 

 \he drop after passing through 2,000 feet would be ^more 



than an eighth of an inch, and after passing through 

 4,000 feet a quarter of an inch, and so on. So that in 

 passing through 8,000 feet of such cloud, it would acquire 

 a diameter of half an inch. 



The fact that raindrops never attain the size of large 

 hailstones is explained as being due to the mobility in the 

 case of large drops of the surface tension of the water, 

 by which alone the drop retains its form, to withstand the 

 disturbing force of the air rushing past ; when the drop 

 reaches a certain size, therefore, it is blown in pieces like 

 the water from a fountain. 



The origin of drops and stones is then discussed — why 

 some of the particles in a cloud should be larger than the 

 others, as it is necessary for them to be in order that they 

 may commence a more rapid descent. A cloud does not 

 always rain ; and hence it would seem that in their 

 normal condition the particles of a cloud are all of the 

 same size and have no internal motion, and that the varia- 

 tion of size is due to some irregularity or disturbance in 

 the cloud. 



Such irregularity would result when a cloud is cooling 

 by radiation from its upper surface. The particles on 

 the top of the cloud being more exposed would radiate 

 faster than those below them and hence they would con- 

 dense more vapour and grow more rapidly in size. They 

 would therefore descend and leave other particles to form 

 the top of the cloud. In this way we should have in 

 embryo a continuous succession of drops. 



Eddies in the cloud also form another possible cause of 

 the origin of drops and stones. 



D'ALBERTIS'S EXPEDITION UP THE FLY 

 RI VER, NEW G VINE A 



RECENT letters from Sydney announce the successful 

 results of Signor L. M. D'Albertis's expedition up the 

 Fly River, and that he was shortly expected back in 

 Sydney, 



The following letter from him to Dr. George Bennett 

 has been published in the Sydney newspapers of October 

 13:- 



'* Dear Doctor,— I have written a letter to the Com- 

 mittee, necessarily very brief, as I have but little time 

 and a very scanty supply of paper. I am satisfied with 

 the collection I have made, not for the number, but for 

 the quality. I have four species of birds of paradise 

 {Paradisea), the P. raggiana, the P. apoda, the twelve- 

 wired bird of paradise {Sehiicides alba), the king bird of 

 paradise {Cincinurns regius), and the rifle bird {Epimachns 

 magnificus). I got a perfect adult specimen of a casso- 

 wary, which I think is Casuarius bicarunculatus ; also 

 the Dasyptihis pecqueti, a new genus of Ptilofis, and a 

 splendid new species of Gracula, and several other small 

 but very interesting birds. I have seen many birds 

 which are not included in the avifauna of New Guinea, 

 as the Pelecanus conspiclllaiics, the Jabiru {^Mycteria 

 australis) and the pygmy goose {Ncttapus pulchdlus). 

 Among my fishes I have some fine and large species. 

 Of reptiles I have very few except a water snake, 

 which I hope will be something extraordinarily new. 

 Among my insects I have some fine Coleoptera, but 

 the season was not very favourable for them. I expect 

 to have about five hundred species of dried plants and 

 between twenty and thirty of living plants, collected far 

 in the interior, many of which I did not get after- 

 wards. I hope Mr. C. Moore will be satisfied, as 

 I have some fine crotons and palms among them, 

 also some ferns with variegated leaves, orchids, and 

 several other plants with variegated or spotted foliage, 

 &c., from the very centre of New Guinea. I hope Mr. 

 Moore has sent to Somerset some Wardian cases, so that 

 the plants may not be destroyed by the sea breezes during 

 the passage to Sydney. I much regret that I cannot send 

 you any specimens, but I have not a box to pack them 



