April 5, 1894] 



NA TURE 



543 



THE SATELLITE OF NEPTUNE. 



''PHE planet Neptune is now in the constellation Taurus, a 

 ^ little to the north-east of Aldebaran; so the following free 

 translation of a paper on its satellite, read by M. Tisserand to 

 the Socieie Astronomique de France in February, and reprinted 

 ill the March number of L' AstronotJiie, is of interest at the 

 present time. 



Less than a month after Qalle discovered Neptune^ in the 

 place assigned to it by Le Verrier, Lassell suspected the exist- 

 ence of a small satellite, and confirmed his suspicion in 1847. 

 This body is very faint, beini:; of the fourteenth magnitude, and 

 a large telescope is required in order to see it. According to 

 Pickering's photometric observations, its size is about the same 

 as that of our moon, hut it is 12,000 times further removed from 

 us, and hence the light we receive from it is very dim. 



It is well known that the satellite is in retrograde motion 

 round Neptune, in the same way as the satellites of Uranus. In 

 this respect these two planets on the borders of the solar 

 system strikingly differ from the others. Comparing Neptune 

 with other planets, it would be expected that he would possess 

 more than one satellite, but though many scrutinies have been 

 made with powerful telescopes, particularly that at Washington, 

 no one has found a new attendant. 



Neptune's moon is not troubled by the motions of companion 

 satellites, so it ought to present a movement of great simplicity, 

 rigorously realising the geometrical movement considered by 

 Kepler. In fact, some astronomers have proposed to use the 

 satellite as a means of testing the uniformity of certain move- 

 ments in the planetary system. The body would constitute a 

 clock of marvellous precision, and with nothing apparently to 

 put it out of order. Accumulated observations have, however, 

 brought to light a singular fact with regard to the satellite's 

 orbit. Five or six years ago, Mr. Marth pointed out that obser- 

 vations made from 1852 to 1883'showed that the orbit was 

 being slowly displaced in a certain direction, its inclination to 

 the plane of Neptune's orbit during this period of thirty-one 

 years having increased by about five degrees — an amount too 

 great to be ascribed to errors of observation. What is more, 

 the observations made by H. Struve with the great refractor at 

 Pulkova, during the last ten years, confirm this variation, both 

 as regards its direction and amount. This being so, the ques- 

 tion arises as to the cause of the disturbance. 



There can be no hesitation in attributing the change to the 

 oblateness of the planet. The amount of polar compression 

 has not yet been determined by direct measurement, and it will 

 doubtless escape detection for some time to come. This is be- 

 cause the disc of Neptune only subtends to us the small angle of 

 about two seconds of arc, and if the oblateness were, say, l/ioo, 

 the ellipticity of the disc would be beyund our perception. 



But in order to account for the changes established by obser- 

 vation, it is necessary to take other matters into consideration. 

 If the plane of the orbit of the satellite coincided with the 

 equator of the planet, there would be no reason why this coin- 

 cidence should not be maintained indefinitely. It seems, how- 

 ever, that the two planes are inclined at a certain angle, and it 

 can be demonstrated that in this case the orbital plane must 

 be displaced with respect to the equatorial one, while the 

 angle between the two remains constant. 



If the poles of these two planes are supposed to be projected 

 upon the celestial sphere, the former will move uniformly 

 round the latter in a circle, and by the accumulation of obser- 

 vations for two or three centuries, the position of this circle 

 could be very accurately determined. The centre of the circle 

 would be above the north pole of the planet ; so by this means 

 it becomes possible to determine the direction of the polar axis 

 — a datum which, as we have seen, cannot be determined 

 directly. The facts at present at the disposal of astronomers 

 are insufficient for the purpose of doing thi-;. It appears 

 probable, however, that the angle referred to is from twenty to 

 twenty-five degrees, and the oblateness less than i/ioo. Prof 

 Newcomb, without going into detailed calculations, has assigned 

 the same cause to the phenomenon. 



The fifth satellite of Jupiter, discovered by Prof Barnard 

 in 1893, ought to exhibit a similar change to that undergone 

 by Neptune's attendant. It does not appear that the four 

 larger Jovian satellites are able to disturb the new one in an 

 appreciable manner ; in this case, moreover, the large oblate- 



' Neptune was locked for and found by Galle on September 23. 1S4 j ; 

 the satellite was discovered by Lassell on October 10 of the same year. 



NO. 1275, VOL. 49] 



ness of Jupiter must be taken into consideration. But the 

 oblateness produces another effect. It may not modify the 

 position of the orbit of the satellite because this small body re- 

 volves in the pi ine of the planet's equator, but it may cause the 

 orbit to turn in its plane, and calculations show that it ought to 

 produce a complete turn in about five months. If, therefore, 

 this orbit is not exactly circular, but ever so little eccentric, a 

 time must come when the satellite must appear at a greater dis- 

 tance from the east than from the west limb of the planet, and 

 this is what Prof Barnard has actually observed. But seventy- 

 five days after these distances must be reversed, for the greater 

 distance should then be from the west limVj. It is to be hoped 

 that future observations will decide this point. The effect 

 referred to ought also to bs shown by the satellite of Neptune, 

 though it is much less pronounced than the change of the 

 orbital plane ; nevertheless, its determination will not be long 

 delayed. 



SCIENCE IN THE MAGAZINES. 



CCARCELY a month passes but what we have to notice 

 ^ astronomical articles in the magazines, a fact which 

 testifies to the interest taken in all that appertains to star- 

 gazing. The Century contains an article of this kind, entitled 

 "A Comet-Finder," by Mr. F. W. Mack, and recounting the 

 ways and work of Mr. W. R. Brooks, well known among 

 astronomers as the discoverer of numerous comets. The title 

 of the article is rather misleading, for while it refers to the man, 

 it is apt to be confounded with the instrument he uses. From 

 the article it appears that Mr. Brooks was born at Maidstone, 

 in 1844, and went to America when he was thirteen years old. 

 In 1870 he settled in Phelps, sixty miles from Syracuse, where 

 he became the village photographer. Like many other notable 

 astronomers, Mr. Brooks made his own. telescopes. His first 

 comet was discovered in 1881 with a five-inch reflector con- 

 structed by himself, and shortly afterwards he made the nine- 

 inch silver-on-glass Newtonian instrument, with which, up to 

 1888, he discovered ten comets. Mr. Brooks then removed to 

 Geneva, N.Y., to take charge of the Smith Observatory, which 

 possesses a two-inch refractor. During his residence there he 

 has discovered eight comets, four of them within less than one 

 year. The total number of his discoveries is now nineteen. 

 The article is illustrated by a photograph of Mr. Brooks, and 

 views of some of the comets found by him. Under the title, 

 " Driven out of Tibet," Mr. W. W. Rockhill describes 

 an attempt made by him in 1891-92 to pass from China 

 through Thibet into India. Mr. G. E. Waring discusses the 

 different methods of sewage disposal, under the heading 

 "Out of Sight, out of Mind." He favours an irrigation 

 system similar to that used at Wayne, Pennsylvania. From an 

 excellent article on "Forest Legislation in Europe,"by Mr. B. E. 

 Fernow, we obtain the following facts. In Germany, contrary 

 to general opinion, laws regarding the use of private forest 

 property are less stringent than among other nations who have 

 paid attention to the matter. In Prussia, which represents 

 two-thirds of Germany, private forests are absolutely free from 

 governmental influence. In Saxony no State control exists. 

 Other States differ in their law., regarding forest property. Of 

 the private forests, seventy per cent, are without any control 

 whatever, and thirty per cent, are subject to slight supervision. 

 In Austria, however, the status of forest legislation is very 

 different, a strict supervision being exercised not only over 

 forests owned by communities, but also over those belonging to 

 private individuals. To ensure a rational management of the 

 forests, the owners of large areas must employ competent 

 foresters whose qualifications satisfy the authorities, opportunity 

 for the education of such being given in eight schools of forestry. 

 In Hungary and Italy also the control of private forest pro- 

 perty is vested in the State. In Russia, until lately, liberty to 

 cut, burn, destroy, and devastate forests was unrestricted, but 

 in 1888 a law came in force which, to some extent, put an end 

 to this liberty of vandalism. The Russian Government now 

 sustains twenty-four schools of forestry. A federal law was 

 passed in Switzerland in 1876 giving the federation control oyer 

 the forests of the mountain region, embracing eight entire 

 cantons and parts of seven others, or over one million acres of 

 forest. The employment of educated foresters is obligatory, 

 and to render this possible, courses of lectures to the active 

 foresters are instituted in the eantons. There is also an 



