388 



♦ KNOWLEDGE • 



[Dec. 28, 1883. 



and from 10 to 20 yards can be covered with a single pull 

 of the handles. 



Tlie new Coventry Rotary Tandem is a " Sociable," made 

 on the same lines as the single machine, but with a second 

 seat and pair of pedals and chain wheel, all of which can 

 be removed in a few minutes so as to convert the machine 

 into a single when required. It is very light for a Sociable, 

 the weight being about 9.") lb. It is an exceedingly fast 

 machine, but would require rather careful riding, and is 

 more suitable for two gentleman than for a lady and 

 gentleman. Still a lady could ride on the front seat, and, 

 as there are two steering-handles, a gentleman riding 

 behind could steer. 



ASTRONOMICAL COLLISIONS.* 



By Prof. C. A. Young, Princeton, N.J. 



THE universe of space is, in one sense, fuU and crowded. 

 Everywhere our telescopes find stars ; and for every 

 one we see there are probably hundreds, and even thou- 

 sands, too faint or small or far away to reach our senses. 

 And of this countless host of worlds not one is at rest, but 

 all are rushing through space, free and unbridled, with 

 velocities far exceeding anything in the range of our ter- 

 restrial expei'ience. Our swiftest cannon-shot takes more 

 than three seconds to go a mile, but the stars and planets 

 seldom move slower than five miles a second, and some of 

 them go two hundred. The veriest snail among them would 

 overhaul a' rifle-ball in the same sort of way that the rifle- 

 ball would overtake a receding freight-train. Is there 

 not, thru, danger of coLisions'? Have such collisions ever 

 occurred ; and, if so, with what consequences ? 



Now, in the first place, it is only in a sense, and rather 

 a Pickwickian one, that space can he called crowded. True, 

 stars are to be seen in every direction ; but this is because 

 our vision reaches so unimaginably far. If we restrict 

 our consideration to bodies of respectable magnitude, — a 

 hundred miles in diameter, for instance, — we ought rather 

 to say that space, instead of being crowded, is almost in- 

 conceivably empty. Between any star or planet and its 

 nearest neighbour, lie usually desolate distances of millions, 

 or even millions of millions, of miles. If we assume, what 

 is probably an under-estimate, that the region commanded 

 by our telescopes is so vast that light takes a thousand 

 years in coming to us from its outer confines, and, further, 

 that the number of its stars is a thousand millions (the 

 number visible with our largest telescopes is usually esti- 

 mated at about sixty millions), then we find that the 

 average distance from star to star must be about nine 

 millions of millions of miles. This is not quite half 

 the actual distance from the sun to our nearest stellar 

 neighbour (a Centauri), according to the received value 

 of its parallax ; but it is a distance which quite defies 

 the power of human comprehension. If two stars at 

 such a distance were rushing straight toward each 

 other with a speed of ten miles a second, it would 

 require nearly .30,000 years to bring them together. 

 Represent the sun, as in Sir John Herschel's familiar illus- 

 tration, by a globe 2 ft. in diameter, so that oranges, 

 cherries, peas, and pins'-heads would stand for the diS'erent 

 planets at distances ranging from 80 ft. to two miles, then, 

 on that scale, a Centauri would be 8,000 miles away. 

 Obviously, there is plenty of elbow-room in the universe, 

 and the chance of two stars accidentally jostling each other 

 is far less than that of the collision of two bidlets in mid- 

 air over a battle-field. Still the possibility remains. 



* From the North American Bevievj. 



There is a prevalent impression that the immunity from 

 collision between the heavenly bodies depends mainly on 

 their being held in orderly orbits by central attraction; and 

 it is true that their orbital constraints make it quite im- 

 possible for the playlets to collide with each other, or even 

 to approacli each other very closely. But a very impressive 

 picture is sometimes drawn of what would liappen if the 

 earth, for instance, should some time free herself from the 

 bonds of gravitation — how she would fly off into space 

 (which is true, of course), and soon dash herself to pieces 

 against some other wandering world (which is not true at 

 all, or at least not probable). Most likely, if gravitation 

 were suddenly to cease, the earth would travel for millions, 

 not of years, but of ages, without ever encountering any- 

 thing more important than meteors such as are now con- 

 tinually falling upon her surface. And these meteoric 

 encounters, even, would probably be far less frequent than 

 at present, since there is reason to ."-uppose that such bodies 

 are much more numerous in the neighbourhood of the sun, 

 and of other great stars, than in the rest of space. 



Evidently, however, the attractions between a wandering 

 star and others lying near its path must considerably in- 

 crease the chances of collision, though not to so great an 

 extent as perhaps might be supposed. A body entering 

 our system at a distance from the sun one hundred times 

 as great as the radius of the earth's orbit, and moving with 

 no more than the velocity natural to a comet at that dis- 

 tance (about 2-^ miles per second), would not strike the sun 

 unless its path were directed within less than half a degree 

 (2.3.'-i) of the sun's centre ; and if its velocity were greater, 

 the aim would have to be still more accurate to score a hit. 

 If bodies were shot into space at random from the position 

 and with the velocity indicated, only about one in eighty- 

 six thousand would hit the sun. Were it not for the 

 sun's attraction, however, the proportion would be twenty 

 thousand times smaller still — only one in seventeen hundred 

 millions. 



It is obvious, therefore, that encounters between heavenly 

 bodies of considei-able magnitude must be extremely rare. 

 There is not in all the astronomical record a single certain 

 instance of such an occurrence. And yet it may confidently 

 be asserted that some time or other in the past such events 

 must have happened, and some time in the future will 

 happen again ; simply for the reason that, in a sufficient 

 length of time, the most improbable things, if only not im- 

 possible, are sure to come to pass. Now, it is almost 

 certain that the stellar motions are not such as to render 

 collisions impossible : the planets, it is true, as has been 

 said before, can never run into each other, because of the 

 size and nature of their orbits, but, so far as can now be 

 ascertained, nothing of the sort holds good for the motion 

 of the stars. We already know enough about some of their 

 so-called " proper motions " to be quite sure that, as a fact, 

 they are not travelling in any simple curves around any 

 common centre, and also that some stars are flying through 

 space with such velocities that no attraction, such as can 

 reasonably be supposed to act in the premises, can control 

 them. Nor does theory, any more than observation, indi- 

 cate a structure of the stellar universe at all analogous to 

 that of the planetary system. Among the stars there is no 

 central dominance and no imperial power, but we have to 

 do with a republic of comparatively free and independent 

 members. 



Sir William Thomson is therefore fully justified in 

 writing : " It is as sure that collisions must occur between 

 great masses moving through space as it is that ships, 

 steered without intelligence to prevent collision, could not 

 cross and recross the Atlantic for thousands of years with 

 immunity from collision." Undoubtedly, the chance of an 



