March 4, i 



NATURE 



421 



Once upon a time, hundreds of thousands of years ago, 

 this comet was traveUing in outer space, among the fixed 

 stars, too far away to be attracted by the sun. What I 

 mean by this outer starry space may be told by the help 

 of the pictures I have shown you. In them the earth's 

 distance from the sun is lo inches, and the comet's longest 

 range about 5 feet. Upon the scale of these figures only 

 a few of the nearest fixed stars, perhaps two or three only, 

 would be in the State of Connecticut. In this starry space 

 the comet was travelling. What had happened before I 

 do not try to guess. How, when, by what changes, its 

 matter came together, and had become solid, I do not 

 know, nor whether, in fact, it had not always been solid. 



In the course of time its path and the sun's path through 

 space lay alongside of each other, and the sun drew the 

 comet down toward itself. If the comet had met no 

 resistance as it ran around the sun, whether from the 

 ether that fills space, or from the sun's atmosphere, and 

 if it had not come near any of the planets, it would have 

 gone off again into outer space whence it came. Some 

 such cause robbed it of a little of its momentum, and it 

 could not quite rise out of the sun's controlling force, but 

 it came around again in an elliptic orbit to remain thence- 

 forth a member of the solar system. It may or it may 

 not then have been a great comet, like Donati's (in 1 858). 



It was probably a small one. It may have made its 

 circuit of the sun in tens of years or in tens of thousands. 



At some time, probably in the early historic ages, it 

 came near the huge planet Jupiter. When it had gone 

 out of his reach it had just momentum enough left to go 

 around the sun in its present orbit of 6| years. It went 

 away from Jupiter an entire and single comet. As it 

 came near the sun, his burning heat acting upon the cold 

 rocky body of the comet cracked off and scattered in 

 every direction small angular bits. At the same time a 

 very thin vapour, shining by its own light, was set free. 

 To this vapour both comet and sun had an unaccountable 

 repulsion. It was driven off first by the comet every 

 way. But soon that which was sent toward the sun was 

 driven back again, and it went streaming off into space to 

 form the comet's tail, a process ably set forth by Prof. 

 Norton. 



This matter which made the tail of the comet never got 

 back. It had, moreover, nothing whatever to do with the 

 meteoroid stream. The meteoroids are solid fragments. 

 To them the sun, at least, had little repulsion. The comet 

 was so small that perhaps the force with which a boy can 

 throw a stone would have sent the bits of stone entirely 

 off the comet, never to come back. Those which were 

 shot forward from the comet near P (Fig. i) went up 

 along the orbit with greater velocity and rose higher from 

 the sun than the comet did near D. Having a longer road 

 to travel, they took a longer time to come around to P in 

 each circuit. On the other hand, those bits which were 

 shot backward followed the comet with less velocity and 



could not quite rise to D, and so having a shorter road to 

 go over came sooner back to P, gaining on the comet at 

 each circuit. Thus the stream grew longer slowly, and 

 new fragments being thrown off at each circuit, the 

 meteoroid stream grew in length to its hundreds of 

 millions of miles. .At times, the main comet has broken 

 into two or more parts, giving us the double comets of 

 1845 and 1852, the Pogson comet of 1872, and the double 

 meteor stream of November 1872. 



THE yAVIGABLE BALLOON^ 

 IVT RENARD, captain of the Chalais-Meudon navig- 

 -'■*-'• • able balloon, has presented to the French Aca- 

 demy of Sciences a report of the experiments made 

 with that balloon last year. Before starting on a fresh 

 campaign in 1885, it was found necessary to make certain 

 modifications in the construction of the balloon, affecting 

 the ventilator, voltaic piles, commutators, &c. To measure 

 the velocity of the balloon, an anemometer, the registra- 

 tions of which would be too strong, seeing that the spiral 

 is placed in front, was impracticable. There was no in- 

 convenience, on the other hand, in the use of an aerial 

 log. A balloon of gold-beater's skin, 120 litres in capa- 



. Ascena on du 2b Aout 

 . Ascension du Tl Sept 

 . Ascension du 23 Sept. IB''^ m2 

 Echelli 



Fig. I. — Map of the journe>:> of the La Frathe balloon 



city, was accordingly filled in part with common gas, so 

 as to keep exactly in equilibrium in the air. This balloon 

 was attached to the central extremity of a bobbin of silk 

 thread just lOO metres in length. The slightest effort is 

 sufficient to unroll this bobbin when the central thread 

 is drawn. The other extremity of the thread is 

 wound round the finger of the operator. To obtain a 

 measurement of speed the balloon is let go, when it 

 quickly flies to the rear, and, on reaching the end of its 

 line, conveys a perceptible indication of the fact in the 

 finger holding the thread. The instant of its departure 

 and that of the twitching sensation in the finger at its 

 terminus are marked on a chronometer counting tenths 

 of a second. Although the force transmitted to the small 

 balloon during the unwinding of the thread is very slight, 

 it is yet necessary to take account of it. Repeated trials 

 in a closed place showed that the little balloon swerved 

 7 metres per minute, or o'l 17 metre per second, under the 

 influence of this light effort. If, then, / be taken as the 

 time in seconds elapsing in the process of unwinding, the 

 way traversed by the navigable balloon during the opera- 



' From La Xaturt\ 



