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THE POPULAR SCIEKCE MONTHLY.— SUPPLEMEXT. 



the fragment would revolve as a planet in an ellip- 

 tic orbit." 



This statement is not very satisfactory, because 

 the velocity of a cannon-ball, depending consider- 

 ably on circumstances, is not a definite unit of 

 measurement. The assertion, too, that the frag- 

 ment would become a comet is open to exception, 

 and nothing is said about the least velocity neces- 

 sary to free the expelled body from the earth. 

 Probably the velocity of a cannon-ball was taken 

 by Lagrange at about 500 yards per second, that 

 being a fair velocity for a 68-pounder at the date 

 of his paper. A velocity, then, exceeding a can- 

 non-ball in the proportion of 156 to. 1, would be 

 about 44 miles a second. Now, for a body ex- 

 pelled from the earth to travel as a retrograde 

 comet, it must be sent backward with a velocity 

 equal to the earth's in her orbit (about 18^ miles 

 per second), increased by the proper velocity for 

 a retrograde comet, about 25£ miles per second, 

 or 44 miles per second in all. This agrees, then, 

 with Lagrange's result. But he seems to have 

 been led from the real subject of inquiry to prob- 

 lems which are only matters of curiosity. The 

 fragmentary planets of Olbers's theory move nei- 

 ther as advancing nor as retrograde comets. 

 Leaving, then, Lagrange's paper, as not very much 

 to the point, if rightly represented by Grant, we 

 note simply that the velocity necessary to expel 

 from the earth a fragment of her mass, in such 

 6ort that it would not be drawn back, would 

 amount to about Y miles per second, or, say, about 

 twenty-five times the velocity of a cannon-ball. 



But again, the expulsion of a fragment, and 

 the explosion of an entire planet, are processes 

 very different in their nature. If a fragment 

 were expelled, the entire mass of the earth would 

 recoil with a motion bearing the same kind of 

 relation to that of the fragment which the recoil 

 of a very heavy cannon bears to the motion of 

 the ball. If a cannon were not heavier than the 

 ball, the cannon would be driven back as rapidly 

 as the ball would be expelled, though frictional 

 resistance would bring it sooner to rest. Again, 

 when a shell at rest bursts, the fragments are 

 driven outward on all sides, with much smaller 

 velocities than any one of them would have if the 

 entire charge of powder acted upon it, the rest of 

 the shell being in some way restrained from mov- 

 ing. We see, then, that for a planet to explode 

 into fragments which thereafter should be free 

 to travel independently around the sun, the ex- 

 plosive force must enormously exceed what would 

 be necessary in the case of a single fragment ex- 

 pelled as a projectile is expelled from a gun. 



When we consider, further, that the frame of 

 the earth is demonstrably not the hollow shell 

 formerly imagined, but even denser at its core 

 than near its surface ; that, moreover, it is not 

 formed of rigid materials, but of materials which 

 under the forces to which they are subject are 

 perfectly plastic and ductile, it seems incredible 

 that under any conditions which appear possible 

 our earth could be shattered by an explosion. 

 Prof. Newcomb, of Washington, in an able paper 

 on this subject, remarks on this objection that, 

 " since the limits of our knowledge are not ne- 

 cessarily the limit of possibility, the objection is 

 not fatal, and it is difficult to say what weight 

 ought to be attached to it ; " and, as many of 

 our readers will remember, Sir W. Thomson, one 

 of the greatest mathematicians living, has not 

 thought the arguments against the possible or 

 probable shattering of a planet sufficiently weighty 

 to prevent the theory from being entertained 

 that one world may be peopled from the seeds of 

 life brought to it by the fragments of another 

 which had exploded. Yet it may fairly be said 

 that if the destructive explosion of a planet is pos- 

 sible, it is utterly improbable ; and that absolute- 

 ly nothing is at present known to us which sug- 

 gests even the bare possibility of such a catas- 

 trophe. 



Yet the theory that a planet which had been 

 traveling between Mars and Jupiter had burst 

 into fragments, had a much more probable ap- 

 pearance in Olbers's time than it has at present ; 

 for the four asteroids first discovered traveled on 

 orbits not differing greatly as to their mean dis- 

 tances, which are as the numbers 236 (Vesta), 

 267 (Juno), 211 (Ceres and Pallas). When as- 

 teroids began to be discovered which traveled 

 nearer to the sun than Vesta, and much farther 

 away than Ceres and Pallas, the explosion theory 

 was shown to be improbable. When, further, 

 the actual paths of these multitudinous worlds 

 came to be examined, the theory was found to be 

 utterly untenable. More recently still a circum- 

 stance noted by the ingenious American astrono- 

 mer, Kirkwood, has pointed to another theory as 

 extremely probable. 



The history of the successive discovery of the 

 various members of the asteroidal family, though 

 not without interest, would be little suited to 

 these pages. A few details, however, may be 

 mentioned here as illustrating the general char- 

 acter of the search. 



We have seen that Hencke engaged in 1830 

 in the search for a fifth asteroid. On the even- 

 ins of December 8th he observed a star of the 



