IO 



HARDWICKE'S SCIENCE-GOSSIP. 



chiefly of loose ashes, with sides of an inclination 

 between 36 and 50 . The guide generally conducts 

 visitors over this loose and unstable foothold, so that 

 as fast as one proceeds one returns half the distance. 

 This fact we soon saw ; we therefore forsook the regular 

 track for an old lava stream, which, although very 

 rough and uneven, afforded a better foothold than the 

 loose ejecta we had left. In the course of an hour 

 and a half our path lay through vast clouds of steam 

 issuing beneath our feet, produced by the subterranean 

 heat evaporating the moisture that had fallen in the 

 night. Mounting at last the edge of the crater, 

 there was presented to our eyes a scene, so fierce, so 

 wild, that a mind could hardly conceive the existence 

 of such without seeing it in reality. 



Here was an amphitheatre-like cavity filled by 

 recent lava, which had overflowed the northern edge 

 and swept down the side of Vesuvius, towards the 

 Atrio del Cavallo. In the centre of the fiery lake 

 rose a cone of about 30 feet in height, built up of 

 fragments of pumice and lava, that on an average 

 of half a minute, were blown (white hot) a hundred 

 feet or so into the air, accompanied by vast columns 

 of acid vapours and gases, and loud rattling ex- 

 plosions, such as are produced by a line of musketry. 

 This was followed by the rattle of the falling pieces 

 on the side of this cone, thereby adding to its size. 

 This cone bears somewhat the same relation to the 

 crater of Vesuvius as the latter mountain does to the 

 pre-historic crater of Monte di Somma. Around it 

 in process of construction and activity were scattered 

 three or four fumeroles, which resemble gigantic 

 sugar-loaves, being covered by incrustations of 

 common salt, sulphide of potash and other sublimates, 

 and from whose summits issued in a rhythmical 

 manner aqueous vapour, and, apparently by the 

 smell, hydrochloric acid. 



We carefully descended the sides of the crater on to 

 the cooled crust of lava. This in many places was 

 cracked and fissured, and looking down one of these 

 cracks we could see the red-hot liquid trachytic sea, 

 upon which we were really floating, producing a feel- 

 ing of the advantages of life assurance companies. 

 Now with caution we direct our path to one of the 

 fumeroles, on which could be obtained some beau- 

 tiful sublimates. Here, really floating on a lake of 

 liquid fire, standing in vast clouds of almost suffocating 

 vapour, not ten yards from the gigantic chimney of 

 an active volcano belching forth showers of hot 

 stones which it was necessary to evade, hearing loud 

 rolling-like thunder beneath our feet, we recalled to 

 our minds the imaginary visit of Dante to Hades under 

 the guidance of Virgil. The scene fascinated our 

 imagination, and produced a profound feeling of awe 

 of man's feebleness compared with the gigantic 

 efforts of nature, and of his greatness in comparison 

 with his ancestors, who, unable to comprehend the 

 scientific explanation and laws under which such 

 effects were and are produced, were obliged to create 



deities of but slight superhuman power to account 

 for phenomena which we now understand. 



The lava as it flows appears to all intents and 

 purposes like liquid asphalte, in the condition it 

 is poured from the caldrons in process of paving our 

 streets, except that it is incandescent. As it flows, it 

 cools on the surface, leaving a crust of sponge-like 

 rock, generally known as scoria;, or it forms for itself 

 an arched channel through which it flows as in a 

 tube. 



The guide shows some interesting experiments j 

 first he forces his stick into the stream and pinches- 

 off a piece of the hot pasty mass. Into this he 

 squeezes a coin and laps over the edges as if it were 

 dough (i.e. not with his fingers) ; it is allowed to cool 

 and then broken open to show the imbedded coin 

 much oxidised. Tongs with their opposed surfaces 

 engraved are made to pinch a piece of pasty rock 

 and so form a medallion. This .Jast flow from 

 Vesuvius is of exceeding vitreous texture, and con- 

 tains an enormous number of crystals of leucite, 

 which crystallize out before the lava is solid, and 

 thus give to its cooled surface the appearance of 

 dough full of currants. This obsidian-like variety,, 

 I believe, is an uncommon product for this volcano. 



We well filled our bags with specimens ; in a 

 quarter of an hour we had descended and were at 

 the observatory ; here we took to our horses and 

 carriages, very tired, but well pleased to return to 

 Naples, having enjoyed a delightful day. Of the 

 minerals, and something about them, more anon. 



CELESTIAL PHOTOMETRY. 

 By John J. Plummer, M.A., F.R.A.S. 



THE measurement of the distance of the sun front 

 the earth, perhaps the most troublesome pro- 

 blem of practical astronomy, is difficult, not from any 

 inherent intricacy in itself, but merely from the fact 

 that a very wide gap has to be passed, that the know- 

 ledge of tlie length of a comparatively short line is 

 the only available datum from which we must infer that 

 of a very long one. It is precisely similar in the 

 matter of the brilliancy of the sun. There would be 

 no difficulty in comparing its light with a terrestrial 

 standard, but it so greatly transcends any artificial 

 light that we may employ that our ingenuity is sorely 

 tested in effecting a measurement or comparison. 

 On the other hand, the light of the stars falls so much 

 short of that of our usual standards of photometric 

 measurement that a difficulty of like character has again 

 to be encountered. The moon, alone of heavenly 

 bodies, is easily comparable with artificial lights, and 

 may therefore fairly engage our attention first. 



There is no more convenient artificial light witli 

 which to compare the moon than the sperm candle, 

 made to burn 120 grains of wax per hour, which has- 

 long been in use for photometric purposes. The light 



