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Planetary Research and Discovery. By W. T. Lynn, B.A.,
Public Health and Mortality. By CHARLES MACKESON, F.S.S.
Abstracts of Important Acts of Parliament (continued) —
2. Finance and Taxation:
Bankers' Books Evidence
3. Law and Justice :
Royal Style and Titles
Registry and Photographing of Criminals
4. Trade and Manufacture:
Industrial and Provident Societies
5. Local Government:
Poor Law Amendment
Cruelty to Animals
Wild Fowl Preservation
Pollution of Rivers
6. Miscellaneous :
Abstracts of Parliamentary and other Official Documents:
1. Finance, Taxation, and Currency.
2. General Commerce and Navigation
3. Trade and Agriculture.
4. Population, Public Employment, and Vital
5. Crime, Pauperism, and Accidents.
6. Education, Science, and Art
9. Social Economy
Chronicle of Events and Occurrences (1875-76).
Necrological Table 1875-76
GENERAL INFORMATION ON SUBJECTS OF ASTRONOMY, PUBLIC HEALTH, CURRENCY,
SCIENTIFIC INSTRUMENTS, ART EXHIBITIONS.
PLANETARY RESEARCH AND DISCOVERY.
THE recent rapidity of planetary discovery has been so great that it is difficult for books of astronomy to keep up with it: and it is only by consulting the current periodicals that any one can be sure that he is au fait with its actual state. It appears to us, however, that our readers may be glad to have a collected view of the progress of these discoveries from its first commence
Now the generic term planet contains two divisions of bodies: those denominated primary planets, which move round the sun, affected only in a small degree by the attraction of others (which merely produce trifling irregularities or perturbations in their motions); and those called secondary planets or satellites which move round the sun as attendants on one of the primary planets, so that their motions with respect to the latter are in fact in the nature of circulating in an orbit round it as it does round the sun. There was a time when these two classes would have been described as having another fundamental difference, in great comparative superiority of bulk in the first division, but this is no longer the case, since the discoveries of the present century have revealed the existence of a very large number of primary planets, all much smaller in size than a large portion at any rate of the secondary.
The primary planets may be divided into three groups. Mercury, Venus, the Earth, and Mars, are nearer the sun than the rest, and are all not very different from each other in size and density, the diameter of the largest (the Earth) being about 2 times that of the smallest (Mercury), whilst the ratio of the densities is very small indeed. Then comes the large group of minor planets, consisting of a number (now known to be at least one hundred and sixty-nine, and probably many more remain to be found, whilst others may be too small ever to be seen by us at all) of very minute bodies, none exceeding about 200 miles in diameter, and the great majority very much less; these move
round the sun in orbits, which are generally much more eccentric than those of the larger planets, and inclined at much greater angles to the planes of the latter, all of which do not differ extravagantly from each other. Finally, we have the four planets farthest from the sun, Jupiter, Saturn, Uranus, and Neptune, which are very considerably larger than the first group, the diameter of even the smallest (Uranus) being about 4 times that of the earth, whilst Jupiter's diameter (the largest of all), is 11 times the earth's; the densities of these four (and the fact is remarkable) are much smaller than those of the planets nearer the sun, those of Jupiter and Uranus being about a quarter of the earth's, or nearly the same as the sun's, whilst those of Saturn and Neptune are not much more than half that quarter.
The secondary planets or satellites (so far as they are known) consist of one moving round our earth, four revolving round Jupiter, eight round Saturn, four round Uranus, and one round Neptune. Owing to the great distance of Uranus, and still more of Neptune, it is extremely likely that there may be others circulating round them which have not been discovered. We shall not allude to these bodies further than by giving the following particulars of their discoveries, which have of course been made since the invention of the telescope. The satellites of Jupiter were first seen by Galileo (though a claim, untenable as we believe, has been set up for priority by Simon Marius, or Mayr) in the beginning of the year 1610, and were nearly the first fruits of telescopic research. The satellites of Saturn have in recent years been honoured with names to obviate confusion which had arisen from their numbering in order of discovery not corresponding with that in order of distance from Saturn. In the latter order they stand Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Hyperion, and Japetus. Of these, Titan is by far the largest, and was the earliest discovered, being detected by Huyghens, whilst sojourning in France, on the 25th of March, 1655. Japetus, Rhea, Tethys, and Dione, were discovered by the elder Cassini, who, on the establishment of the Royal Observatory at Paris in 1671, took charge of it at the instance of Colbert after resigning his Italian appointments to do so. There he detected the four satellites in question; the first two in October 1671 and December 1672 respectively, the last two in March, 1684. It was reserved for Sir William Herschel to discover the two satellites, Mimas and Enceladus, nearest to Saturn, which he did by the aid of his powerful instrumental means in the autumn of 1789; Enceladus on August 28th, and Mimas on September 17th. The discovery of Hyperion took place in our own time, and was made simultaneously on the night of September 19, 1848, by Mr. Lassell at Liverpool, and by Professor Bond at Cambridge, in the United States. With regard to the satellites of Uranus, it is well known that their number has given rise to a good deal of discussion, but we do not propose to enter into this with any detail here. The existence of four has been clearly established,