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» KNOWLEDGE ♦ 



[May 18, 1883. 



ON THE FORMATION OF COMETS' 

 TAILS. 



By a. C. Ranvard. 



]F the repulsive force which drives the matter of a 

 comet's tail away from the sun is due to evaporation 

 of the sulistance of meteoric particles, there must be in 

 space meteoric matter of a kind which does not reach the 

 earth's surface. By far the largest number of the meteoric 

 bodies which have been seen to fall belong to the class of 

 stony meteorites. I make a distinction between the 

 meteoric masses which have been picked up immediately 

 after their fall and the stones and masses of iron collected 

 in museums to which a meteoric origin is attributed, 

 because, owing to differences of hardness and to differences 

 in their chemical constitution, some meteoric masses with- 

 stand the weathering effects of air and water better than others ; 

 and we should therefore expect to find a larger proportion 

 of the harder and better weathering meteorites in museums 

 than amongst the known falls. Iron meteorites are also 

 more easily recognised as of meteoric origin by an ordinai-y 

 observer than stony meteorites, and consequently an undue 

 jiroportion of such iron meteorites ultimately find their way 

 to museums. 



One must not, therefore, go to the catalogues of meteoric 

 bodies collected in the museums of Europe and America 

 to determine the proportion of the different kinds of 

 meteorites existing in space. We shall probably obtain a 

 truer idea of the kind of matter which circulates in the 

 parts of space swept by the earth, if we confine our 

 attention to a smaller number of meteoric bodies whose 

 fall has been observed, Of these, according to the estimate 

 of Dr. Flight, about 90 per cent, are stony meteorites ; the 

 remaining 10 per cent, belong chiefly t>- the nickel iron 

 class, and there are only a very few bodies composed of 

 amorphous carbon and bituminous matter, such as the 

 Kold Bokkeveld, Orgueil, and Alais falls. 



Without pretending to say what would be the tempera- 

 ture on the sun-illuminated side of a dark body in space 

 at the earth's distance from the sun, we may be sure that 

 the heat derived from solar radiation would not be sutii- 

 cient to raise such a body to a temperature of white 

 incandescence, for the moon (which certainly is not enve- 

 loped by an atmosphere of a thousandth part the density 

 of the earth's atmosphere) appears nearly black when it 

 passes into the earth's shadow during a lunar eclipse. The 

 slightly ruddy tint which the moon's disc presents under 

 such circumstances cannot even be due to a cherry-red 

 temperature of the lunar surface, for if this were the case, 

 the ruddy tint would be most visible in the region of the 

 lunar equator, where the sun's rays had been falling nearly 

 vertically ; but the red light is more or less evenly difl'used 

 over the whole lunar disc, and dillers considerably from one 

 eclipse to another, showing that it is due to light which has 

 passed through the earth's atmosphere, and has reached the 

 moon after absorption within the zone around the earth, 

 which an observer on the moon would see under such cir- 

 cumstances as a line of red light fringing the dark disc of 

 the earth, interrupted here and there by clouds and haze in 

 the lower air. 



J will not digress to give an account of the Earl of 

 Rosse's observations as to the temperature of the lunar 

 surface, or of other considerations which may be urged to 

 show that the temperature of the sun-illuminated side of a 

 dark body not surrounded by an atmosphere, and situated 

 at the earth's distance from the sun, lays somewhere 

 betwcr^n 300'' and 500° Fahr. For my present purpose, it 

 will be suilicieat if we take it as proved that a dark body 



would not be raised by such dii-ect radiation to a tempera- 

 ture of white incandescence. No doubt, all bodies are 

 volatilisable if exposed to a suflliciently high temperature j 

 but I know of no meteorite, whether of the Kold Bokkeveld, 

 or of the iron or stony class, which could be driven into 

 vapour by temperatures that would not suffice to make the 

 body glow with a white heat. 



Comets frequently develop tails at distances from the 

 sun considerably greater than the earth's distance. The 

 great comet of 17l'9, when in perihelion, was more than 

 four times the earth's mean distance from the sun, and it 

 showed during a period of more than five months an exten- 

 sive nebulosity, though the heat due to solar radiation 

 must have been less than one-sixteenth the heat derived 

 from solar radiation at the earth's distance. 



Mr. A. S. Davis, and those who think with him that the 

 repulsive force which drives the matter of a comet's tail 

 away from the sun, is due to evaporation of the solid 

 matter of meteoric bodies, may reply Viy suggesting that 

 possibly blocks of ice travel in space with the meteoric 

 bodies that reach the earth's surface, and that the ice- 

 blocks are evaporated by the heat developed in passing 

 through the earth's atmosphere. There are some theorists 

 who have even suggested that hail comes from outer space. 

 To the advocates of the meteoric origin of hail, I would 

 reply that if a storm of hailstones came into the 

 earth's atmosphere with a velocity comparable with 

 the planetary velocities with which meteoric bodies 

 usually encounter the earth, the whole sky would be 

 illuminated 'with a dazzling brightness which it is 

 difficult to give an idea of. A single meteor, which 

 enters the earth's atmosphere to a depth of nine or 

 ten miles above the sea level, and, much more, a meteor 

 which reaches the earth's surface, attracts the attention of 

 observers for miles round, even when the fall occurs during 

 bright sunshine ; and a million such meteors, entering the 

 atmosphere at once, would, by their light and sound, 

 dazzle, as well as deafen, the most unobserving of men. 



It is usually difficult to drive from his hobby the 

 thorough-going theoriser, who is always ready with a new 

 supposition, which he does not condescend to found on 

 observed facts ; and the meteoric hail-men reply that it 

 is not necessary that the earth should meet the showers 

 of meteoric haD. The hailstones in space may all be tra- 

 velling in the same direction as the earth round the sun, 

 and with but slightly greater velocity, so that they may 

 come up quietly from behind, and pass through the earth's 

 atmosphere at any rate you please. I would remind them 

 that the earth's attraction is sufticient to give a velocity of 

 seven miles a second to a body which starts from rest at 

 an infinite distance, and though it is possible to conceive 

 of a single meteor falling with a less velocity, showers of 

 meteoric hail, catching up the earth from behind, must 

 enter the earth's atmosphere with a velocity of at least 

 seven miles a second. 



Sir William Thomson has shown that a thermometer, 

 placed in front of a body which moves through the atmo- 

 sphere at the rate of ] 25 feet per second, rises one degree ; 

 with higher velocities the increase of temperatures is pro- 

 portional to the square of the velocity, being four degrees 

 for a velocity of 250 feet, sixteen degrees for a velocity of 

 500 feet, and so on. Consequently, a velocity of seven 

 miles a second would bo sufficient to cause a rise of tem- 

 perature of more than eighty-fi\c thousand degrees, which 

 would be more than suflicieut to drive iron and platinum 

 into vapour, and would cause the moving body to glow 

 with a brilliant white incandescence. 



Mr. Davis, who, I assume, does not advocate the theory 

 of meteoric hail, may say tliat the heat developed in passing 



