'lept. 30, 1875] 



NATURE 



487 



which could be heard forty or fifty miles away? Nay, why 

 should it explode at all ? 



One answer is this. The aerolite comes into our atmosphere 

 from regions in which the temperature, " the cold of space," may 

 range as low as 140° below zero Centigrade ; and though the 

 mass, from the absorption of solar heat, would possess a tempera- 

 ture much above this, it would nevertheless be intensely cold, 

 and consequently more brittle than at ordinary temperatures ; and 

 hence, en its entering our atmosphere, the heat it instantaneously 

 acquires on its outer portion expands this, and tends to tear 

 it away, so as to dissever the exterior from the interior, which 

 continues to be relatively contracted by the intensity of the cold 

 which the aerolite brings with it from space. The consequence 

 is, first, that little bits of the stone spring out all over it, leaving 

 those curious little holes or pit marks which are characteristic of 

 a meteorite ; and eveiy now and then, as the heat penetrates, 

 larger masses split away, of which interesting evidence is afforded 

 by the meteorite, for instance, that fell at iJutsura on May 12, 

 i86r. Fragments of this stone were picked up three or four 

 miles apart ; and by supplementing them by a small piece 

 modelled to fill up one lacuna, one is able to build up again with 

 much certainty the original meteorite, or at least the portion of 

 it represented by the fragments of it which were found. Im- 

 portant portions of this stone are in the British Museum, presented 

 some years ago by the liberality of that invaluable institution, the 

 Asiatic Society of Calcutta. Now, it is remarkable that these 

 fragments, which in other respects fit perfectly together, are, even 

 on the faces of junction, now coated with a black crust. On the 

 other hand, another of these fragments not thus coated fits like the 

 former to a part of the meteorite that was found some miles away 

 rom it, and is also not incrusted at the surface of fracture. Hence 



we can assert that this aerolite acquired after coming into our at 

 mosphere a scoriated and blackened surface or incrustation. The 

 first explosion drove the fragments first alluded to asunder, and 

 these became at once incrusted on their broken surfaces ; but 

 others that were separated afterwards, probably on the last of the 

 three explosions, had not sufficient velocity left to cause their in- 

 crustation in the same manner as was the case with the fragments 

 previously severed. Now, this successive incrustation of the 

 fragments of the meteorite confirms the idea that the disruption 

 of the mass, and the explosions heard for so vast a distance as 

 Goruckpore (some sixty miles), are parts of the same convulsion ; 

 and sixty miles is by no means an uncommon distance for the 

 sound of such a meteoric explosion to be heard. 



The late W. von Haldinger (to whom we are indebted for a 

 collation of the facts and for valuable suggestions bearing on this 

 subject) threw out the notion that what really produced the de- 

 tonation was not the disruption of the mass (which he held not 

 to be a sufficient cause for so loud a report) so much as the col- 

 lapse of the air into a vacuum which, after following the 

 meteorite as it pursued its rapid course, suddenly ceased to exist 

 as the velocity of the meteorite became practically reduced to 

 zero. 



But it still would remain to be explained why at one time 

 more than another this collapse of the vacuum should take place, 

 or how it could be repeated ; of this, however, a sufficient ex- 

 planation would seem to be afforded by the actual bursting 

 asunder of the meteorite from the cause before assigned, since 

 this explosion, by disturbing the conditions on which the per- 

 sistence of the vacuutn depends, would permit the collapse of 

 the air and consequent detonation. 



(7a be continued.') 



OBSERVATIONS ON A REMARKABLE FOR- 

 MATION OF CLOUD AT THE ISLE OF 

 SKYE* 



T^HE resistance offered by the earth's surface to the wind is 

 •^ known to reduce its velocity and to cause deviations in its 

 direction both horizontal and vertical, as well as to retard 

 the progress of the storm itself This friction to which aerial 

 currents are subjected is probably least for a surface of water 

 such as the sea — greater for plains of loose sand, where, as in 

 the Nubian deserts, lofty sand pillars are produced — and greater 

 still where the surface is immoveable, as in the case of solid 

 land ; but the greatest resistance of all is due to the obstruction 

 offered by rugged hills and lofty mountain-ranges. 



In an account of the Morayshire easterly storm of September 

 1871 ; published in the Scottish Meteorological Journal, I sug- 

 gested that the great amount of rainfall which fell on that occa- 



sion at and near the Morayshire coast, and on the sea-coasts of 

 the counties of Fife and East Lothian which also fronted this 

 storm, was due to the sudden increase of friction which the wind 

 encountered when it reached the land. The in-shore stream of 

 air being checked by the unyielding nature of the shore, even 

 though it was, as in this case, of no great elevation, would form 

 a pillow of obstructed or perhaps nearly stationary air, which 

 would produce vertical deflection on the strong currents coming 

 in from the sea. The stream of air thus projected upwards to a 

 height where the temperature is lower would be condensed into 

 vapour and rain. 



This sudden change of resistance to in-shore winds is probably 

 one of the causes of the well-known peculiarity of seaside 

 climates. 



On the 27th July last, about 11.30 a.m., when in the steamer 

 of the Northern Lighthouses off the Sound of Harris, I saw a 

 beautiful example of the genesis of c'ouds — due, however, not to 



a low foreshore, but to hills of about 900 f.el high. The sky was 

 perfectly clear, with a steady but very slight breeze from the 

 S.W., which came straight upon the south-western extremity of 

 the Island of Skye, distant about twelve miles from the ship. 

 A small portion of the most southerly projection of the island, 

 which was considerably lower than the more inland parts, was 

 perfectly free from vapour, but at a short distance Inland from 

 the shore, there was an abrupt face of hill, from the top of which 

 there rose a very slender column of white vapour which gradually 

 expanded as it ascended into the air, presenting exactly the 

 appearance of the escape of steam from the spiracle of a volcano. 

 The cloud thus formed not only extended as far as the northern 

 extremity of Skye— itself a distance of twenty-eight miles— but 



* By Thomas Stevenson, F.R.S.E. 



was visible as a well-defined stratum of cloud for a long distance 

 beyond Skye, so that its whole length must have considerably 

 exceeded forty miles, beyond which distance it became more 

 diffuse and attenuated. Had I not known to the contrary, I 

 should undoubtedly have believed that what I saw was due to 

 volcanic eruption. 



The vapour caused by the lower temperature of the atmo- 

 sphere at the level of the top of the bluff face was obviously 

 carried away by the breeze gradually as it was fi)rmed, thus pro- 

 ducing by a continuous process of generation the long extent 

 of cloud which I have described. This fact shows that clouds 

 may be due to deflections produced by irrejjularitics on the 

 earth's surface far remote from the place where we actually see 

 them. I may mention, in proof of the steady nature of the 

 breeze and of the entire absence of any vertical disturbance in 



