Feb. 1 8, 1886] 



NA TURE 



367 



nature. But when an explosion subsequently occurred in 

 the mine, and traversed the whole length and breadth of 

 the workings, which were known to be practically free 

 from fire-damp at the time, producing the most disastrous 

 effects, the fallacy of the conclusions drawn from the 

 experimental results was rendered abundantly evident. 



In the whole of my papers on this subject, and most 

 pointedly m my last article on coal-dust explosions pub- 

 lished in Iron, in the year 1878, I have carefully indicated 

 that a difference was to be expected in the behaviour of 

 dust ignited under the two conditions named. It has 

 therefore been with feelings of considerable surprise that 

 I have observed members of the French, English, and 

 German Mining Commissions, and others who have 

 investigated this subject since the publication of my first 

 paper, one after the other pronounce some very decided 

 opinions as to the relatively subordinate part which coal- 

 dust plays in a colliery explosion, while at the same time 

 they were neglecting to take into account this very simple 

 and yet all- important element. W. Galloway 



VESUVIAN ERUPTION OF FEBRUARY 4, 1886 



THE rent that was formed on May 2, 1885, in the 

 upper part of the great cone (NATURE, vol. xxxii. 

 P- 55) gave issue to lava until December 25. A small 

 quantity again issued between January 2 and January 5, 

 1886, after which no more made its appearance till this 

 new outburst. In consequence of the rise of level of the 

 magma in the chimney, the cone of eruption has grown 

 very much during the last month. 



On February 4, at about 8 p.m., lava broke forth at the 

 foot of the old crater ring of 1881-2 at a point bearing 

 from the main vent about 10° W. of N., traversed the 

 crater plain, which is here very narrow, in a somewhat 

 oblique direction, and ran down the slope of the cone 

 between north and north-north-east. The lava soon 

 reached the foot of the cone, but even up till midday to- 

 day, when I left the mountain, it had not yet commenced 

 to cross the Atrio del Cavallo. The eruption took place 

 from probably the same dyke that gave rise to that of 

 January 9, 1884. 



To-day, February 6, the lava bubbles up like a spring 

 at the foot of a hill, and flows for some distance in a kind 

 of trough which it has raised on each side of itself above 

 the level of the crater plains. After a short distance it 

 enters one of its own tunnels to reappear again at some 

 distance. It was very interesting to watch its welling, and 

 from time to time the bursting of a steam bubble as big as 

 a bucket, which would throw up splashes against the 

 imperfect arch at the immediate exit. These splashes 

 partly adhered to the roof and partly fell, drawing out the 

 suspended portion into irregular strings, illustrating the 

 formation of the stalactitic lava that is so common in 

 lava fumaroles. 



The chloride crusts in the neighbourhood were uncom- 

 monly rich in copper, so that my boot-nails were thickly 

 plated with that metal. 



The lava makes its appearance at about 100 yards from 

 its entrance in the tunnel near the main spring, although 

 it is now divided into two streams. The eastern, which 

 is the largest, is i metre broad ; I plunged a stick in to 

 the depth of i metre, but the shortness of the stick and 

 the great heat prevented me from touching the bottom. 

 The current ran at the rate of i metre per 6 seconds, 

 which, making allowance for viscous drag at the edges 

 and bottom, will give an output of at least 360 cubic 

 metres per hour, or at least 17,280 cubic metres during 

 the 48 hours up to 8 o'clock this evening. The more 

 western stream was 50 centimetres broad, over r20 

 metres deep (as far as I could reach with my stick), and 

 flowed at the rate of i metre in 8 seconds. Giving a loss 

 of 2 seconds of speed from drag at sides and bottom, we 

 have an output, for 48 hours, of 10,368 cubic metres. 



The two streams together would, therefore, have afibrded, 

 since the commencement of the eruption, 27,648 cubic 

 metres. 



As the altitude of the lateral outlet is much more than 

 that of May 2, both on the night of the eruption and the 

 following one, the volcano showed \}ai. first stage of activity 

 as judged by the appearance of the main vent. 



This winter the mountain has been covered by snow 

 several times, and to-day it extends down nearly to the 

 level of the observatory. During our ascent we had to 

 walk through a stratum of about 8 inches, though much 

 thicker in the drifts. Two-thirds of the crater and part 

 of the cone of eruption were also covered. 



I should have sent news yesterday, but, on attempting 

 an ascent, 1 was driven back by wind, rain, and mist. 



Naples, February 6 ' H. J. JohnstoN-Lavis 



TIDAL FRICTION AND THE EVOLUTION OF 

 A SATELLITE 



A PAPER by Mr. James Nolan has recently appeared 

 which is devoted to an adverse criticism of my 

 views concerning the effects of tidal friction as a factor 

 in the evolution of the moon and earth. 



The title of the pamphlet, " Darwin's Theory of the 

 Genesis of the Moon,"' shows, I think, that the author 

 has misconceived the scope of my work. It was not 

 supposed that the investigation threw light on the actual 

 mode of genesis of the moon, but was rather applicable 

 to the subsequent history of the moon and earth. Mr. 

 Nolan attributes to me views as to the condition of the 

 moon immediately after her birth which do not appear a 

 just interpretation of my writings, and although it might 

 have been well to use more guarded expressions in 

 some passages, the justice of his condemnation of the 

 whole theory cannot be admitted. He sums up his case 

 by the three following propositions : — 



"(i) That the moon could not have existed bodily so 

 near the earth as the greatest initial distance fixed. 



" (2) That in any form possible there she could not 

 have receded by the agency assigned — tidal friction. 



"(3) That, if a modification be made by allowing her 

 to have separated at a greater radius than that corre- 

 sponding to a period between 2 and 4 hours, the moon 

 would be no longer traceable to the earth's present surface 

 on which condition the theory has been founded.'' 



The first of these propositions is interesting, and I have 

 to thank him for drawing my attention to it. 



When a small satellite revolves about a planet with a 

 certain proximity, the sum of the centrifugal and tidal 

 forces may be such as to overbalance the gravitation 

 towards the centre of the satellite. When this is the 

 case, the satellite cannot exist as a single mass. The 

 complete solution of the problem, concerning which Mr. 

 Nolan adduces certain elementary considerations, is of 

 extreme difficulty. At present I do not wish to go into 

 this question, but shall consider the point on another 

 occasion. It may, however, be admitted that the moon 

 could not subsist as a single continuous body with its 

 surface in contact with the earth. 



On p. 4 he quotes a passage from the abstract of one 

 of the papers {Proc. R.S., No. 200, 1879), which must be 

 surrendered ; it is as follows : — 



"The coincidence is noted in the paper that the 

 shortest period of revolution of a fluid mass of the same 

 mean density as the earth, which is consistent with an 

 ellipsoidal form of equilibrium is 2 hours and 24 minutes ; 

 and if the moon were to revolve about the earth with this 

 periodic time, the surfaces of the two bodies would be 

 almost in contact with one another." 



Now, since 1879 it has been shown by Sir William 

 Thomson that the ellipsoidal form referred to could not 



' Geo. Robeitsonand Co., Melbourne, Sydney, Adelaide, and Brisbane, 

 1885. Pp. 16. 



