October 1, 1895.] 



KNOWLEDGE. 



231 



found silence and in the unmitigated glare of the sunlight 

 we should see the molten rock creeping sluggishly from 

 the lips of the crater, and in the place of the explosive 

 escape of volumes of steam the surface of the lava flow 

 would merely be agitated by the bubbling out of what 

 would immediately become a frosty garment of snow and 

 carbon-dioxide. It would ba little more terrific than the 

 squeezing of paint from a tube. The forcible and sustained 

 ejection of scoriae and ashes on a terrestrial volcano is due 

 largely to the efi'ervescent escape of superheated steam 

 from the molten magma ; but if the lunar surface is, as is 

 generally supposed, somewhere near the absolute zero of 

 temperature, then the isotherm of the freezing point of 

 water must be some considerable distance beneath the 

 crust, and the boiling p3iut isotherm still deeper. In 

 which case the expansive force of the contained water may 

 be much less in a lunar than in a terrestrial crater, and 

 indeed it may be insufficient to spray up or even vesiculate 

 the more viscous lava flow. On the other hand, however, 

 the feebler gravitationalenergy of themoon and the absence 

 of a superincumbent atmosphere woidd enable a much 

 smaller expansive force to project masses to a considerable 

 altitude, and a far smaller force of upheaval to rupture a 

 much greater thickness of overlying crust. The lunar 

 eruption would be therefore, one may think, more of the 

 nature of one violent explosion like the discharge of a gun, 

 and nothing more, rather than the sustained pyrotechnic 

 display of a terrestrial outbreak. 



Taken altogether, these considerations point to the 

 conclusion that a lunar eruption, if such a thing is still 

 possible, would consist essentially of two phases ; the 

 first, the very transient one of breaking through the crust, 

 would eject any obstacle in the vent to an enormous 

 altitude, the ejected rock or rocks, in a more or less 

 pulverized condition, raining back immediatiUj about the 

 volcanic vent, and then might follow an inconspicuous 

 and noiseless bubbling and guttering out of snow and frozen 

 gas, and (less probably) lava within the lips of the crater. 

 The former scarcely more than the latter could be expected 

 to be visible from this planet. 



But it must be borne in mind that these are purely 

 speculative suggestions, involving finally the groundless 

 implication that the moon has differences of internal 

 temperature, and so some lingering vestiges of internal 

 energy. There is plausibility in the belief that at the 

 absolute zero of temperature matter will have lost all its 

 inter-molecular energy, and, among other things, that it 

 will not bs able to contract further. If we assume that 

 not only the unillumiuated surface of the moon, but its 

 very centre also, has sunk to that final static state, then 

 the volcanic forces due ii the contraction of a coaler 

 exterior upon a warmer nucleus do not come into play. 

 But along another line the grounds for anticipiting lunar 

 changes are less hypothetical. Taere mast still be super- 

 ficial displacements due to the expansions and contractions 

 which the monthly passage of the solar heat-wave must 

 cause, and there is not only parioiic heating and cooliag 

 of the lunar surface, but since no matter is perfectly rigid, 

 there must also be a tidal deformation due to the sun's 

 attraction. These influences, at any rate, whatever we 

 may think of the volcanic possibility, »ius( produce tiltings 

 and instabilities, and in the tremendous impact of 

 meteorites— for the moon, unlike the earth, has no 

 pneumatic protection from such jars— we have a force 

 more than adequate to start landslips and overthrow the 

 tottering summits of cliffs. Altogether there is plentiful 

 a priori ground for denying that the moon is indeed an 

 immutable dead world beyond all further indignities of 

 change. 



And this is not merely an a priori proposition, for 

 about twenty years ago there is every reason to suppose 

 that a black spot appeared near Hyginus, and in 1866 a 

 certain amount of discussion centred about the crater 

 Linne. The floor of Plato has also been suspect, but 

 without any very decisive results. Assuredly there can be 

 no more promising field of observation for the amateur 

 astronomer in possession of a fairly powerful telescope 

 than a detailed study of some definite portion of the lunar 

 disc, and an exhaustive comparison with tlie accumulating 

 collection of photographic charts that have been made 

 during the past decade. To see the side of some mighty 

 crater suddenly slide and crumble into ruin, or some gaping 

 chasm opening in a dazzling slope of white, has so far been 

 the lot of no terrestrial observer. Yet it may be that 

 already such change has happened in the field of some 

 watching telescope, and only escaped observation because 

 the eye that watched was set against the expectation of 

 change. And even as this is written some happy mortal 

 may be detecting the faint stirring, the scarcely perceptible 

 movement that marks the still hving forces that have so 

 far been hidden from our eyes. But the chances are that 

 whatever changes may be proceeding will be detected in 

 a less dramatic way — by the systematic measurement and 

 comparison of photographic charts extending over a 

 considerable period of years. 



THE SIZE OF THE SOLAR SYSTEM. 



By J. E. Gore, F.E.A.S. 



AS my readers are aware, the solar system consists 

 of a number of planets revolving round the sun as 

 a centre, and of subordinate systems of satellites 

 revolving rouad the planets, or at least round some 

 of them. Our own earth is one of these planets, 

 the third in order of distance fi'om the central luminary, 

 which forms the common source of light and heat to all 

 the members of the system. In addition to the planets and 

 satellites, there are also some comets which form permanent 

 members of the solar system. Some of these comets 

 revolve round the sun in very elongated orbits, while the 

 planets revolve in nearly circular orbits. A consideration 

 of the absolute size of this planetary system and its relative 

 size compared with that of the universe of stars, or at least 

 the universe visible to us, miy prove of interest to the 

 general reader. 



To determine the size of the solar system it is, of 

 course, necessary in the first place to ascertain the dimen- 

 sions of the planetary orbits with reference to some 

 standard, or unit of measarem3nt as it is termed. Tue 

 uuit of measurement adopted by astroaomsrs is the suq's 

 distance from the eirth. As the earth is the third planet 

 in order of distance from the sun, this distance is, pf course, 

 an arbitrary unit. We might take the mean distance of 

 Mercury from the suu as the uuit, but as we refer all our 

 measurements to terrestrial standards, anithe diameter of 

 the earth is used in the measurement of the suns distance, 

 it is found more cons'euient to take the sun's distance from 

 the earth as the standard of measurement for the solar 

 system and the distance of the stars. 

 " The relative distances of the planets from the sun have 

 been determined by astronomical observations and are 

 represented approximately by the following figures, the 

 earth's mean distance fi-om the sun being taken as unity : 

 Mercury S87, Venus 0-723, the earth 1-, Mars 1-528, 

 the mmor planets 2-08 to 4-262, Jupiter 5-203, Saturn 

 9-538, Uranus 19-183, and Neptune 30-055, or taking 

 the earth's mean distance from the sun as 1000, the 



