274 



HARDWICKE'S SCIENCE-GOSSIP. 



[Dec. 1. 1866. 



SHOOTING-STARS. 



rrniERE are few who witnessed the meteoric dis- 

 -*- play on the morning of November 14th, but 

 must have felt a wish to know something of the 

 nature of the objects which swept in such numbers 

 across the heavens. The very fact that the display 

 had been predicted — that these apparitions, silently 

 but swiftly streaking the sky, were known to be 

 due on that day, and at that hour — gave an interest 

 and significance to the phenomenon, which would 

 have been wanting to a fortuitous display, however 

 brilliant. 



The questions which one would like to have 

 answered are such as the following : — How far off 

 are these bodies ? What are they made of ? How 

 large are they ? How heavy ? — and so on ; questions, 

 to some of which it might seem hopeless to seek for 

 an answer, so far at least as meteors which do not 

 reach the earth are concerned. Let us see whether 

 astronomers have anything to say on these points. 



As respects the distance at which shooting-stars 

 appear and disappear, the following results have 

 been attained : — Father Secchi, of the Collegio Ro- 

 mano, Prof. Newton in America, and Mr. Alexander 

 Herschel in England, estimate the average height of 

 shooting-stars at the moment of appearance to be 

 74|, 73|, and 70 miles, respectively ; and the average 

 height at disappearance to be 49£, 50i, and 54 miles, 

 respectively. These results accord so closely as to 

 leave no doubt that on an average, and within very 

 narrow limits of error, 73 miles and 52 miles are the 

 distances separating the earth from shooting-stars 

 at the moments of appearance and disappearance, 

 respectively. 



It is not very easy to determine the apparent 

 size (or, thence, the true size) of a shooting-star, on 

 account of the suddenness of its appearance and the 

 rapidity of its motion. We may note, however, 

 that it has been estimated that meteorites (which 

 belong to the same family as shooting-stars) vary in 

 diameter from about 100 feet to 13,000 feet. 



The velocity with which shooting-stars move has 

 been determined in many instances. It results from 

 these estimates that they move at an average rate of 

 about 35 miles per second — a velocity more than 

 twice as great as that of the earth in her orbital 

 motion round the sun. In some cases, however, 

 they attain a much greater velocity — even, it has 

 been asserted, to 100 miles per second. 



It would not be easy to explain in brief space the 

 method by which the weight of a shooting-star may 

 be determined. It will be sufficient to say that 

 Mr. Alex. Herschel, by a method founded on just 

 scientific principles, has shown that the average 

 weight of shooting-stars is very little over two 

 ounces. A similar method applied to the largest 

 meteor seen in 1863 showed its weight to be no 

 less than 224 lbs. Hence it may be concluded that 



shooting-stars have very much smaller dimensions 

 than their " big brothers," the meteorites. 



Science has enabled the physicist to learn the 

 constituent elements of shooting-stars. The most 

 wonderful aid to physical research ever yet in- 

 vented, the spectroscope, serves us in this inquiry. 

 We thus learn that some of the shooting-stars are 

 incandescent solids, but that the majority are re- 

 duced by intense heat (at the time and during the 

 interval of apparition) to the gaseous state. Con- 

 spicuous among the substances thus converted into 

 gas is the metal sodium. It is to be noted, further, 

 that the salts of this metal have been found in the 

 meteorites which fell in August, 1865, near Aumale. 

 So far as the August meteors are concerned, we 

 have the opinion of Mr. A. Herschel, that their con- 

 dition is exactly that of " the flame of a spirit-lamp, 

 newly trimmed, and largely dosed with a supply of 

 moistened salt." Meteorites which reach the earth 

 are found to contain many elements known on earth, 

 but no new elements, and only one or two new com- 

 pounds have ever been discovered in them. 



Observation has shown, also, a very remarkable 

 circumstance concerning the apparent paths fol- 

 lowed by shooting-stars. It is found that these 

 paths, prolonged backwards, all pass (on any the 

 same occasion) through or near one point on the 

 celestial sphere. No matter whether this point be 

 just rising above the horizon, or passing its highest 

 point on the southern meridian, or setting, the same 

 peculiarity is observable. But for different showers, 

 that is, for showers occurring at different seasons, 

 different " radiant points " (so astronomers have 

 named these centres of divergence) are exhibited. 



I have left to the last the most wonderful fact of 

 all ; namely, the recurrence of showers at certain 

 epochs. We have, for instance, the shower of 

 August 9th or 10th, the shower of November 13th 

 or 14th, and the shower of October 20th or 21st. 

 These are, in some respects, the most remarkable 

 showers. But many, perhaps, will be surprised to 

 learn that the number of known recurring epochs of 

 star-falls is no less than fifty-six ! Each of these is 

 well distinguished from other showers by its ra- 

 diant-point, by the character of the stars composing 

 it, by the extent and brilliancy of the display, by its 

 regular recurrence year by year, or by regular or 

 irregular intermittences. It maybe noted, in this 

 connection, that the nights from the 8th to the 15th 

 of December are likely to exhibit to the patient 

 observer trainless meteors, individually much finer, 

 perhaps, than the shooting-stars of November 14th. 



Three theories have been suggested in explana- 

 tion of the phenomena we are considering. It has 

 been suggested, first, that shooting-stars are mere 

 atmospheric phenomena, and that even aerolites and 

 meteorites are generated in the upper regions of 

 air. Secondly, it has been supposed that we owe 

 the visits of shootirg-stars to volcanoes in the moon, 



