March 23, 1899] 



NA TURE 



489 



If in connection with diurnal waves we wish to record 

 sunshine, or to note the rise and fall of stars or distant 

 objects as seen through a telescope at the time of large 

 earth waves, the station should command, especially in 

 an east and west direction, a fairly extensive horizon. 



Inasmuch as an observer may, as a means by which 

 "air tremors" can be destroyed, require in one of his 

 rooms a copious ventilation with a minimum of damp- 

 ness, a precaution of some importance is not to ignore 

 the hygrometric conditions of a locality. 



A good site having been found, the remaining require- 

 ments for a seismological observatory are small. All 

 that is necessary is a small one-storied structure. It 

 should contam one or two large rooms in which to place 

 some half-dozen instruments, and three small rooms to 

 be used respectively as an office, a workshop and a dark 

 room. 



In Italy there are fifteen observatories of this order, 

 and a \ery large portion of the work is to record move- 

 ments of the earth's crust, which can be equally well 

 recorded in England. At Strassburg, which is as free 

 from earthquakes as any town in England, a seismo- 

 logical observatory, costing 3500/., with an annual grant 

 for maintenance of 275/., is being erected. Austria and 

 Germany are establishing stations, whilst the great work 

 which for years past has been carried out in Japan is too 

 well known to require restating. 



In conclusion, when we consider that the observations 

 made at a seismological laboratory are connected with 

 those made by the meteorologist, the geologist and the 

 astronomer, that they suggest problems to the elastician, 

 shed light upon perturbations of magnetic needles, are of 

 direct importance to the cable engineer, and in the inter- 

 pretation of certain telegrams, and that in many other 

 directions they are of value both scientifically and 

 practically, it seems strange, especially in the face of the 

 hearty co-operation we have received from abroad, that 

 this country is yet without a definite centre at which 

 these observations can be carried on. 



John Milne. 



SATURN'S NINTH SATELLITE. 

 (~\^ Saturday last, March 18, the astronomical world, 

 ^-^ somewhat recovering from the excitement incident 

 to the discovery of the remarkable asteroid now named 

 Eros, was again pleasantly surprised by the news of 

 another " find," distributed by telegram from the Central 

 Astronomical Bureau at Kiel. This time it is the planet 

 Saturn which supplies the feature of interest, in that an 

 addition to its already numerous family of attendant 

 satellites has been discovered by Prof William H. 

 Pickering, assistant astronomer at Lowell Observatory, 

 Flagstaff, Arizona. The name of this station will be 

 familiar to all in connection with the many notable ob- 

 servations of the planet Mars which have been made 

 there by Mr. Lowell, its director, with the 24-inch refractor. 

 Most of this work is so delicate as to need the best con- 

 ditions for seeing, and it is only the extremely favourable 

 situation of this observatory which has rendered them 

 possible. This is probably to be attributed to the extreme 

 transparency of the air consequent on the high altitude 

 above the sea-level. 



The new satellite has been run to earth, as it were, by 

 photography. On examination of four photographs of 

 Saturn, Prof Pickering found traces on each of a very 

 faint object, the behaviour of which led him to consider 

 it to be a satellite of the planet. The little stranger is 

 estimated to be of the 15th magnitude, so that it is un- 

 likely that it would ever have been discovered by visual 

 observation, even in the huge instruments now at the 

 disposal of our leading astronomers. Measurements of 

 the coordinates of its position from the four plates have 

 NO. 1534, VOL. 59] 



furnished the data for computing its period or time of 

 revolution round the parent planet, and this is found to 

 be about seventeen months. This indicates that it will 

 take its place as the outermost of the nine satellites, the 

 period of Japetus, the furthest from Saturn of the known 

 ones, being only about 79! days. While the distance of 

 Japetus is 2,225,000 miles, that of the new moon will 

 therefore be about 7,500,000 miles, and this, combined 

 with its extremely slow motion, all tended to diminish 

 the chances of its detection by the usual method of 

 tracking non-stellar objects by the elongated trails they 

 leave on the photographic plate, the stars being shown 

 as symmetrical round dots. 



It is interesting to note how the gradual discovery of 

 the attendants of the various planets has influenced the 

 compounding of the "laws" which from time to time 

 have been found to approximately represent the positions 

 of these bodies in the solar system. From the first dis- 

 covery of Jupiter's four satellites by Galileo in 1610 to 

 the recognition of the already known eight of Saturn by 

 Huyghens, Cassini, and Sir W'. Herschel, no regulai 

 relationship was perceived. When, however, in August 

 1877, Prof Asaph Hall discovered the two moons of 

 Mars, Deimos and Phobos, with the newly-erected 

 26-inch refractor of the United States Xaval Observatory 

 at Washington, it was seen that all the then known 

 satellites were grouped in a geometrical progression, 

 reckoning outwards from the Earth. Thus the Earth 

 had one, Mars two, Jupiter four, and Saturn eight. 

 This seeming regularity was broken by the discovery on 

 September g, 1892, of a fifth satellite to Jupiter by Prof. 

 E. E. Barnard at the Lick Observatory. This last 

 discovery of a ninth satellite for Saturn v.'ill furnish a 

 reason for a new series being formed, as counting front 

 the Earth outward from the Sun, the numbers of satellites 

 to the planets Earth, Mars, Jupiter, and Saturn are now 

 I, 2, 5 and 9 respectively, and these numbers are very 

 nearly proportional to the distances of those planets 

 from the Sun. 



No information is yet to hand as to the diameter of 

 this newly-found member of the solar system. From 

 its brightness it may be from 100-200 miles, but its 

 measurement will be extremely difficult. 



The importance of photography in astronomical 

 research is very well illustrated in the case of this event. 

 Although it might be possible to see the satellite under 

 good conditions, it is easy to understand how many times 

 such an insignificant object might be passed over among 

 so many more prominent ones. Once it has impressed 

 its image on a photographic plate, however, it is caught, 

 and its detection is sure, sooner or later, on complete 

 examination of the negative. Then the possibility of 

 duplication removes all doubt of personal error of any 

 kind. .Another advantage of the photographic plate 

 over the eye is that the longer it is exposed, so much 

 fainter objects will it record ; while, on the other hand 

 the eye only becomes more fatigued the longer it is used 

 in the search. 



It should be instructive to notice how most of the 

 astronomical discoveries of late years hail from across 

 the Atlantic. Whether it is that the love nf science is 

 more generally developed there, or that the liberal en- 

 dowment of a scientific institution is considered the most 

 serviceable way of handing one's name down to posterity, 

 it is certain that in the establishment of the Harvard 

 College, Lick and Yerkes Observatories the American 

 people have placed themselves ahead in astronomical 

 matters ; and there is little doubt that they are well 

 satisfied with the results obtained by means of their 

 liberality. 



A later telegram to the S/andiu'd states that the dis- 

 covery was made with the Catherine-Bruce telescope, 

 an instrument of large aperture and short focal length. 



C. P. Butler. 



