May 18, 1883.] 



KNOVS/LEDGE 



293 



through the earth's atmosphere would melt his cubic mfctre 

 blocks of ice, but I cannot permit him to retire under 

 cover of such a plea. The November meteors enter the 

 earth's atmosphere with a velocity of about 11 miles a 

 second, so that if we assume incandescence to commence at 

 a height of 120 miles, a meteor of the November shower 

 entering the earth's atmosphere perpendicularly, would, 

 if its motion were not retarded by the atmosphere, occupy 

 only about three seconds in reaching the earth's surface ; 

 but, taking into account the diminished \elocity due to the 

 resistance of the earth's atmosphere, a meteor must fall 

 very obliquely if it occupies more than a minute after the 

 iirst instant of incandescence in reaching the earth's 

 surface. With an ice-block, heat would be conducted to 

 the interior more slowly than in the case of an iron 

 meteorite ; and with temperatures such as we are consider- 

 ing, the ice-block would probably lose a thinner skin in 

 passing through the earth's atmosphere than an iron 

 meteorite of the same dimensions. 



The experiments of Professor Graham and the more 

 recent and fuller investigations of Professor Wright show 

 that all classes of meteoric bodies hold gas as a sponge 

 holds water. Whether meteors pick up these occluded 

 gases in distant regions of space and disgorge their contents 

 at the sun when in perihelion is a speculation I will leave 

 to Sir W. Siemens. But laboratory experiments show 

 that the application of heat to meteoric bodies causes an 

 outflow of occluded gas. In space, the gas must pass away 

 from the meteoric masses and be chiefly projected towards 

 the sun with velocities which can be calculated as accu- 

 rately, when the kind of gas evaporated and the tempera- 

 ture of evaporation is known, as the velocities with which 

 the meteoric nuclei are moving can be determined. We 

 ■are not leaving terrestrial analogies when we suppose that 

 a mist forms by the condensation of the less volatile gases 

 at a distance from the nucleus where radiation into space 

 •can take place freely in all directions. The particles of 

 mist will have a velocity away from the nucleus, and they 

 will also be bombarded on the side towards the nucleus by 

 the swiftly-moving molecules of the more volatile gases. 

 Condensation on the surface of the mist particles and 

 evaporation from their sunward sides will be set up and 

 repulsion from both the nucleus and sun will follow. 



The largest outflow of gas will be on the sunward side 

 •of the conietary nucleus, and repulsion from the sun will 

 there act in a nearly contrary direction to the repulsion 

 feoia the nucleus. The mist will rise from the nucleus 

 until brought to rest by the solar repulsion*, and the mist 

 particles wOl then mo\e under the resultant force with a 

 tendency, as I have shown, to aggregation on a hyperbolic 

 sheet which has the sun for one focus and the nucleus for 

 the other. If Jlr. Da\is's theory as to the evaporation of 

 the substance of his meteoric ice blocks were true, there 

 ■woiild be a great loss of substance while the evaporating 



• Professor G. P. Bond, in the volume in which he diecusscs the 

 observations of the great comet of 185S, published as Vol. III. of 

 the " Annals of Harvard College Observatory," gives tables show- 

 ing the gradual rise of successive envelopes from the nucleus, and 

 lie shows that there is a tendency to a dimini.shed rate of ascent as 

 the distance from the nucleus increases. The rate of ascent of the 

 ▼ertex of the envelopes between Sept. 20 and Oct. 20, 1858, varied, 

 according to the estimate of several observers, from 800 to 1,500 

 miles a day. Professor Bond thought that he had good evidence 

 <lerived from certain dark spots upon the envelopes to prove that 

 the material of whicli they were composed remained in the 

 ^cinity of the nucleus for several days, and was ultimately driven 

 off from the cusps on either side of the nuclfeus (see p. 356 of the 

 Tolnme above referred to). The great velocity vritb which the 

 matter of tlie tail is driven away may bo gathered from the rate 

 Tlith which new tails are formed in a different direction as the 

 ■Briena sweeps round the sun at perihelion. — A. C. E. 



I 



masses hung balanced in the coma, and the ultimate 

 velocity of repulsion would consequently Ije less than he 

 has calculated, but the mist particles will continually pick 

 up fresh matter, and evaporate it again towards the sun. 



If the light of a comet's tail was dispersed by particles 

 driven backwards by evaporation of their own substance, 

 we should expect the light from the tail to be most com- 

 pletely polarised towards the fainter end away from the 

 nucleus ; for the light dispersed by small particles is only 

 completely polarised when the disjtorsed light is viewed at 

 right angles to the direction of illumination, and when 

 the diameters of the particles dispersing the light are small 

 compared with the wave length of light Viewed from all 

 other positions, the intensity of the polarisation varies with 

 the magnitude of the light dispersing dust. But no increase 

 of polarisation towards the end of the tails of comets has, 

 as far as I am aware, ever been detected. The polariscopic 

 evidence, therefore, does not point to a gradual decrease in 

 the diameters of the particles dispersing the light towards 

 the fainter end of the tail. 



THE RED SPOT ON JUPITER. 



AT the iirst regular meeting of the American Astro- 

 nomical Society, held in the directors' room in the 

 Brooklyn Academy of Music, on March 5, the topic for 

 discussion was the " Physical Changes in Jupiter." A 

 paper on the " Red Spot on Jupiter" was expected from 

 jlr. S. V. White, the president of the society, and there 

 was considerable disappointment over his inability to be 

 present 



The remarks of the members were confined to the recent 

 changes in Jupiter. Messrs. Parkhurst and Serviss 

 described their observations of the great red spot which 

 made its appearance on Jupiter's disk in the summer of 

 1878, and which within a few weeks has almost entirely 

 disappeared. Others took part in the discussion. 



The general opinion expressed was that Jupiter is a 

 world which is yet in a very early stage of its geological 

 history, and that in the great red spot, and in some other 

 remarkable spots which have made their appearance upon 

 its surface, evidences are seen that the planet either 

 has already a solid or liquid surface, or that the formation 

 of such a surface has begun. Several theories to account 

 for the great red spot, which was upwards of 30,000 miles 

 long by G,000 or 8,000 miles wide, were suggested. 



One theory was that some volcanic action may have been 

 taking place, which threw up into the atmosphere a mass 

 of smoke and erupted materials which formed the red spot. 

 jVnother theory was that the crust of the planet where the 

 spot appeared may have been exceptionally heated, so that 

 tiie atmosphere above it was kept free from clouds. A third 

 theory regarded the red spot as possibly a solidified mass 

 thrust up through its gaseous and liquid surroundings, and 

 forming, perhaps, the nucleus of one of the future conti- 

 nents of the giant planet. The difference between the rates 

 of rotation of the red spot and of the white spot in the 

 southern belt was referred to. The red spot overtakes 

 the white spot once in thirty-four days. 



The society meets on the first Monday in April in the 

 physical laboratory of the Packer Institute. At that time 

 special papers will be read upon the connection between 

 sun spots and terrestrial meteorology.— (Sciex^i/ic American. 



During the past year there were 532 servants of our 

 railway companies or contractors reported as having been 

 killed and 2, -123 injured, in addition to those concerned in 

 accidents to passenger trains. 



