April II, 1878] 



NATURE 



465 



have taken place ? I fear that before his objection can be fairly 

 urged something more definite must yet be known as to the rate 

 of motion of the stars. 



All tliat we are at present warranted to affirm, I presume, is 

 simply that of the comparatively few stars whose rate of motion 

 has been properly measured, none has a motion greater 

 than thirty or forty miles per second, while nothing what- 

 ever is known with certainty as to the rate of motion of the 

 gi-eater numbers of stars. Before we can ascertain the rate of 

 motion of a star from its angular disiilacement of position in 

 a given time we must know its absolute distance. But it is only 

 of the few stars which show a well-marked parallax that we 

 can estimate the distance, for it is now generally admitted that 

 there is no relation between the apparent magnitude and the real 

 distance of a star. All that we know in regard to the distances 

 of the greater mass of the stars is little more than mere con- 

 jecture. Even supposing we knew the absolute distance of a 

 star and could measure its amount of displacement in a given 

 time, still we could not be certain of its rate of motion unless 

 we knew that it was moving directly at right-angles to the line 

 of vision, and not at the same time receding or advancing to- 

 wards us ; and this we could not determine by mere observation. 

 The rate of motion, as determined from its observed change of 

 position, may be, say, only twenty miles a second, while its 

 actual velocity may be ten times that amount. 



By sjiectrum analysis it is true we can determine the rate at 

 which a star may be advancing or receding along the line of 

 sight independently of any knowledge of its distance. But this 

 again does not give us the actual rate of motion unless we are 

 certain that it is moving directly to or from us. If it is at the 

 same time moving transversely to the observer, its actual motion 

 may be more than 100 miles per second, while the rate at which 

 it is receding or advancing, as determined by spectrum analysis, 

 may not be twenty miles a second. But in many cases it would 

 be difficult to ascertain whether the star had a transverse motion 

 or not. A star, for example, i,<X)0 times more remote than 

 a Centauri, that is, twenty thousand billion miles, though moving 

 transversely to the observer at the enormous rate of 100 miles 

 per second, would take upwards of thirty years to change its 

 position so much as i" and 1,800 years to change its position i'. 

 In fact, we should have to watch the star for a generation or 

 two before we could be certain whether it was changing its 

 position or not. And even after we had found with certainty 

 that the star was shifting, and this at the rate of i' in 1,800 

 years, we could not, without a knowledge of its distance, express 

 the angle of displacement in miles. But from the apparent 

 magnitude or brilliancy of the star we could not determine 

 whether its distance was ten times, 100 times, or 1,000 times 

 that of a Centauri and consequently we could form no conjec- 

 ture as to the actual velocity of the star. If we assumed its 

 distance to be ten times that of a Centauri, this would give a 

 transverse velocity of one mile per second. If we assumed its 

 distance to be 100 times that of a Centauri, this would give ten 

 miles a second as the velocity, and if 1,000 times, the velocity 

 of coxirse would be 100 miles per* second. 



As there are but few of the stars which show a measurable 

 parallax and having no other reliable method of estimating their 

 distances, it follows that in reference to the greater number of 

 the stars neither by spectrum analysis nor by observation of 

 their change of position can we determine their velocities. 

 There does not therefore appear to be the shadow of a reason 

 for believing that none of the stars has a motion of over thirty 

 or forty miles per second. For anjrthing that at present is 

 known to the contrary, the majority of them may possess a 

 proper motion enormously greater than that. 



There is, however, an important point which seems to be 

 overlooked in Mr. Phimmer's objection, viz., that unless the 

 greater part of the motion of translation be transformed into 

 heat, the chances are that no sun-star will be formed. It is 

 necessary to the formation of a sun which is to endure for 

 millions of years, and to form the centre of a planetary system 

 like our own that the masses coming into collision should be 

 converted into an incandescent nebulous mass. But the greater 

 the amount of motion left unconverted into heat, the less is the 

 chance of this condition being attained. A concussion which 

 would leave the greater part of the motion of translation untrans- 

 f ormed woidd be likely as a general rule to produce merely a 

 temporary star, which would blaze forth for a few years or a 

 few hundred years, or perhaps a few thousand years, and then 

 die out. In fact we have had several good examples of such 



since the time of Hipparchus. Now, although it may be true 

 that according to the law of chances, coUisioas producing tem- 

 porary stars may be far more numerous than those resulting in 

 the formation of permanent stars, nevertheless the number of 

 those temporary stars observable in the heavens may be perfectly 

 insignificant in comparison to the number of permanent stars. 

 Supix)se there were as many as one hundred temporary stars 

 formed for one permanent, and that on an average each should 

 continue visible for 1,000 years, there would not at the present 

 moment be over half-a-dozen of such stars visible in the heavens. 



James Croll 



The Age of the Earth 



With reference to the ingenious suggestion by Mr. Preston, 

 on the earth's orbit having been practicidly diminished by ethe- 

 real retardation, there are a few other points to be considered. 

 I. That the minor planets could never have passed the major 

 planets, as they would be certainly caught by them during the 

 immense number of revolutions in which their orbits would be 

 nearly equal. Therefore the earth cannot have dropped in from 

 much farther than Jupiter's present orbit ; for if during its revolu- 

 tions it came within one-sixth of the distance from Jupiter that it 

 now is from the sun, it would be mastered by Jupiter. 2. By^the 

 retardation of Encke's comet it seems that if the comet had the 

 same orbit as the earth, its distance from the sun would diminish 

 about ■^-^\^Ts PS'^ year. But for any appreciable lengthening of 

 the earth's life-period, the earth must have started much more 

 than one-tenth farther from the sun than it now is ; that is to 

 say, it must fall in much quicker than at the rate of its present 

 distance from the sun in 10' years. This shows that the indi- 

 yidual portions of Encke's comet must be much more than two 

 miles in diameter, even supposing it to have as great a 

 mean density as the earth, and to consist of a shower of 

 solid meteors. Thus if the earth's history should be length- 

 ened by any important amount from this cause, the nucleus 

 of Encke's comet must consist of a shower of bodies of as great a 

 density as the earth, and of a considerable size, each weighing 

 very much more than 100,000,000 tons. And considering that 

 there must be thousands of such bodies to compose it, the total 

 mass would be greatly beyond what is considered possible. 3. 

 If the earth had drawn much nearer to the sun, the asteroids 

 must have come in from a very much greater distance ; and yet, 

 though they differ greatly in size, ihey are all grouped closely 

 together, whereas we should find then sorted out very much more 

 widely, and a vast quantity of theni retained by Jupiter as 

 satellites. • , .- 



The solar system appears to be really a quinary system of 

 stars ; the major planets being analogous to the sun in their 

 characteristics of density, distances and proportions of satellites, 

 and other elements, the minor planets being the sun's satellites. 

 Thus it is seen that the uniform law of satellites is to regularly de- 

 crease in volume both close to, and farthest from, their primaries ; 

 the series manifestly terminating in asteroids in the case of the 

 sun and of Saturn. 



In the whole of the present discussion of the earth's age, what 

 is the reason why only one out of several different limits is con- 

 sidered? I. The decrease of temperature in the earth. 2. Tidal 

 retardation. 3. The cooling of the sun, which is recognised as 

 being the weakest of the three. 4. A uniform diffusion of tem- 

 perature in the earth, which gives a limit, not for life, but for 

 the separate existence of the earth. The close agreement of the 

 limits of life history given by these first three methods is a very 

 strong argument in favour of each of them ; for if there is even a 

 possibility of I in 5 that each separately is wrong, it would be 

 less chance than i in 100 that the concordance of all three was 

 wrong. 



Is there anything so stable and certain in geologic time- 

 when we remember that levels permanently alter as quickly as 

 ten feet per century— that rainfall (and therefore denudation) 

 depends mainly on the almost unknown changes in the sun's 

 heat, a slight increase of rainfall making much greater rapidity 

 of denudation — and that accumulation of peat and stalactite might 

 well become proverbial for its variability — when all these un- 

 certainties are remembered, is there anything so indubitable as 

 to warrant our throwing all the odium of incorrectness on the 

 cosmical chronology, and seekmg to square it with geological 

 suppositions? W. M. FWNDBRS Petrie 



Bromley, Kent 



