192 



KNOWLEDGE 



[October 1, 1892. 



space," it would be capable of giviup our sun a velocity of 

 255'2 miles per second if it fell towards tbe nebula from 

 an infinite distance, and tbe velocity of our sun in a | 

 circular orbit about such a nebula, situated at a distance ' 

 equal to the distance of a Cciitaiiri, would be 180'4 miles i 

 per second. If tbe nebula contained such a vast mass of i 

 attracting matter we should expect to see many stars in 

 the neighbourhood of the nebula moving across the line of i 

 sight with very large proper motions, for a velocity of 

 100 miles a second across the line of sight at the distance 

 of a. Centanri would give an annual proper motion of 25-5", 

 that is a proper motion more than three times as great as 

 that of 1830 Groombridge, which is the swiftest moving 

 star which has at present been discovered. 



But instead of finding tbe stars in tbe immediate ' 

 neighbourhood of the Orion nebula exhibiting large proper 

 motions, we find tbe stars in this region of the heavens, ! 

 which appear to be associated with the Orion nebula, show 

 hardly any detectable proper motion, and the same remark 

 applies to all the stars connected with the stream of 

 nebuliF which appear to link up the great Orion nebula 

 with the Milky Way. The stars of the Pleiades group, ' 

 which also appears to be connected with the Milky Way 

 and to be surrounded by a very extensive nebula (see 

 Knowledge for May, 1891), also exhibit only small annual ' 

 proper motions, and the same remark applies to the 

 nebulous star a Cijfini, which appears to be associated with 

 the nebulosity of the Milky Way (see Knowledge for 

 October, 18!»1). 



If, instead of assuming the distance of the Orion nebula 

 to be equal to the distance of a <'i'nt<nui, we had assumed 

 its distance to be double as great, the velocities referred to 

 above, of our sun in a parabolic orbit or in a circular 

 orbit about the nebula would need to be doubled. For at 

 twice the distance a nebula subtending tbe same angular 

 diameter would occupy eight times the volume, and, the 

 density remaining the same, its mass would be eight times ! 

 as great, and the periodic time in a circular orbit about \ 

 such a nebula at double distance would be unchanged, ' 

 therefore the velocity in the larger orbit would be doubled. 



That the periodic time is independent of the distance of 

 the nebula will be evident when it is remembered that the 

 square of the periodic time, in any orbit, is inversely 

 proportional to the attracting mass. Therefore, if the 

 central attracting mass be multiplied by eight, the size of 

 the orbit remaining unaltered, the periodic time will be ' 

 reduced in the proportion of 1 to .^l/^- On the other hand, j 

 if it is evident from Kepler's law connecting the squares 

 of the times with the cubes of the distances, that if the 

 attracting mass remains unaltered, and the size of the orbit 

 be doubled, the periodic time will be increased in the ratio of 

 1 to 2\/2 ; therefore, if the size of tbe orbit be doubled, and 

 the attracting mass be multiplied by eight, the periodic 

 time will remain unaltered, I and the velocity in the orbit j 

 will be doubled if we double the distance of the nebula. 



If we assume the distance of the Orion nebula to be \ 

 equal to the distance of a Centauri, that is, taking the 

 parallax of a. Centanri as 0-75", or that it is situated at a 



* In view of what we know of the motions of binary stars about 

 one anotlier, the philosopher who has serious doubts whether the 

 action of gravity extends across interstellar space must have a very 

 highly-developed organ of philosophic doubt. He must be willing 

 to assume that while gravity extends apparently, according to law, 

 across the space which separates the sun and Neptune, and across 

 the probably still wider spaces which separate many of the binary 

 stars, its action ceases or is interfered with in passing across the 

 interstellar spaces which separate the systems within which gravity 

 reigns. 



t The above reasoning only holds when the mass of the sim may 

 be neglected as compared with the mass of the nebiUa. 



distance of 274.900 times tbe earth's distance from tbe 

 sun, and that the mean density of the nebulous matter is 

 one hundred millionth of the density of atmospheric air 

 at the sea level — that is, that the density of the nebula 

 is about Ta 2,T7ro^ooo,oijotl' of ^^^ density of the sun — for 

 water is about 846 times as heavy as an equal volume of 

 air at the sea level, and the density of the sun is about 

 1-444 as compared with water; the mass of the nebula, 

 supposing it to be spherical and to have an angular 

 diameter of 20' would be 330,200 times the mass of the 

 sun, and the periodic time in a circular orbit about such a 

 body at a distance equal to the distance of a. Centauri from 

 the sun would be 281,450 years, which corresponds to a 

 velocity of a little more than 18 miles a second. If we 

 suppose the nebula to be at double the distance of 

 a Centauri the velocity in a circular orbit would be 3G miles 

 a second, and so on, the velocity increasing directly as the 

 distance of the nebula is increased. 



^^'e may probably feel quite sure, from the small observed 

 proper motions in the neighbourhood of the Orion nebula, 

 that its average density does not exceed one ten thousand 

 millionth of the density of atmospheric air at the sea level. 

 This would about correspond to the mean density of the 

 solar nebulous mass, supposing it to have been spherical 

 when its radius was a little more than 107 astronomical 

 units, or when the sun occupied a sphere with a radius of 

 a little more than S.j times tbe distance of Neptune. 



In examining the forms of nebulse we find comparatively 

 few oblate spheroids, such as the hypothesis of La Place 

 assumes. There are many apparently spherical masses, a 

 few spirals and rings, and a great many nebulous masses 

 of irregular form. If the stars we see are of very different 

 ages, and the nebular stage of condensation occupies, as 

 has hitherto been supposed, a very lengthy period compared 

 with the stellar stage, we should expect to see a far greater 

 number of nebular masses than of fully-formed stars, but 

 the number of brightly shining stellar points greatly 

 exceeds the number of nebular masses hitherto discovered. 

 Possibly we are mistaken in supposing that the iaintly 

 shining nebular masses we observe afford ocular evidence 

 of the truth of La Place's bold hypothesis. The nebulje 

 we see have, it seems to me, a greater analogy with the 

 solar corona than with the fiery condensing mists conceived 

 of by La Place ; they are very generally associated with 

 stars, and in some cases the nebulous structure clearly 

 indicates that the nebulous matter has issued from the 

 star, and sometimes from a starless region. The forms of 

 nebula? are certainly in general inconsistent with the theory 

 that stars are condensing from nebulas. 



The dark tree-like structure which extends diagonally 

 from the right lower corner of our plate up towards the left 

 hand upper corner clearly tells the story of matter projected 

 into a resisting medium, and not of nebulous matter slowly 

 condensing under the influence of steady rotation. If the 

 reader will take the trouble to compare two copies of the 

 plate — one a dark print and the other a print in which 

 the nebulosity is whiter — he will recognize several smaller 

 tree-like forms extendmg laterally on either hand from the 

 main trunk of the dark structure, as well as several streams 

 of stars evidently springing from the dark region. 



[The Editor does not hold himself responsible for the opinions or 

 statements of correspondents.] 



THE ATMOSPHERES OF CELESTIAL BODIES. 

 To the Editor of Knowledge. 

 Sir, — In discussing the atmospheres of the moon or of 

 Mars, I think we rely too much on the analogy of our own 



