230 



NATURE 



[July 7, 1892 



of the satellite's shadows as they were projected on the extreme 

 points of the two axes of the spot, the mean giving the position of 

 the spot's centre. In the case of the third satellite, when its lati- 

 tude was — 1° 45' 14", that of its shadow — reckoning from the 

 bottom side of the spot — was - 30° 34' 36". The latitude of 

 the shadow of the second satellite came out to be - 17° 48' 10", 

 and after allowing for the fact that it was projected tangentially 

 on the side of the spot and for the diffraction of the instrument, 

 this value for the latitude of the north side of the spot became 

 - 20° 56' 37". Taking the mean of the values obtained from 

 both satellites, the latitude of the centre was -25° 45' 36", and, 

 with the polar semi-diameter as unity, the magnitude of the 

 spot was o '20297. 



The mean value of the latitude obtained from eleven obser- 

 vations by Denning, Green, Ricco, Williams, Keeler, and Terby 

 was 2i°*5 ± 2°'05, the major and minor axes of the spot being 

 0"555 and 0'i88 respectively. Using the micrometer, the lati- 

 tude, according to Young, amounted to 40° ; while Denning 

 estimated that the major axis embraced an arc varying between 

 29°'3 and 37°-8. 



A Mean Time Sun-dial. — A very ingenious sun-dial, 

 capable of indicating mean time, has been recently invented by 

 Major-General Oliver, the construction of the instrument being 

 undertaken by Messrs. Negretti and Zambra. In an ordinary 

 sun-dial, the time is read off generally by the position of the 

 centre of a shadow, cast by a straight-edged style, on a flat sur- 

 face on which the hours are graduated. The peculiarity of the 

 present instrument is that the time is indicated by the position 

 of the edge of a shadow cast by a " nine-pin" shaped style, with 

 regard to an equatorial circular line. The style is fixed along the 

 diameter of a semicircular arc, which is clamped by means of 

 a screw to a firm stand to suit any latitude ; at right angles to 

 this arc, and also capable of adjustment, is another semicircle, 

 graduated in five-minute divisions. Owing to the change of 

 declination of the sun throughout the year, different parts of the 

 shadow of the style are brought on to the hour circle in such a 

 way that the difference between the time indicated (by the dial) 

 and mean time, or the equation of time, is counterbalanced by 

 the change in position of the shadow, due to the peculiar form 

 of the style. If we start, for instance, on December 24, the 

 readings have to be taken from the shadow of the eastern edge of 

 the lower part of the style in an upward direction, the bulging 

 out of the style counteracting the increase and decrease of the 

 equation of time (which is here positive) until June 14 is 

 reached. Owing to the thickness of the style's axis, a slight 

 adjustment is here necessary when we pass to the other side of 

 the style ; this adjustment is facilitated by placing the twelve 

 o'clock graduation to the western of two marks shown on the 

 vertical circle. This being done, the readings from the shadow, 

 cast now by the western side of the upper protuberance, are 

 taken until the other nodal point on June 14 is reached. At 

 this time also— in fact, four times a year — this slight alteration has 

 to be made. From this latter date until December 24 is reached 

 the same process is repeated, only the respective opposite sides 

 of the style are used in the inverse order. To obviate the 

 necessity of having two styles, which, of course, would have to 

 be the case if the greatest accuracy were desired, owing to the 

 differences in the maxiaium values of the equation of time, one 

 with a mean contour is given : the error produced by this is 

 practically very slight, amounting in time to about one-sixteenth 

 of greatest value of the equation of time — a quantity scarcely 

 appreciable, on account of the lack of sharpness of the edge of 

 the shadow. 



Comet Swift (1892 March 6). — Edinburgh Circular li^o. 28 

 contains a continuation of the ephemeris of Comet Swift (March 

 6, 1892) for the month of July and part of August, from which we 

 make the following extract :— 



Berlin Midnight. 

 1892. R.A. Decl. log A. log r. Br 



h. m. s „ / 



July 7 o 52 36 -F48 o'5 



8 53 33 12-6 0-2391 0-2552 0-17 



9 54 28 24-5 



10 55 20 36 -2 



11 56 10 47-7 



12 56 58 59-0 0*2427 02665 0-16 



13 57 43 49 lo-o 



The brightness at the time of discovery being taken as the 

 unit of brightness, it will be seen that the comet is at present 



NO. II 84, VOL. 46] 



more than five times dimmer than it was in March. In fact, it i* 

 rapidly becoming invisible, and will only be able to be observed 

 with large instruments for another two months or so. Its posi- 

 tion on July 7 will lie to the very southern extremity of the 

 constellation of Cassiopeia, forming nearly an equilateral triangle 

 with I and ir. 



Stars' Proper Motions. — Mr. J. G. Porter contributes to 

 the Astronomical Jottrnal, No. 268, a catalogue of the proper 

 motions of 301 stars, which amount to half a second or more in 

 a year. This list, as he informs us, is -from a still more exten- 

 sive catalogue which he hopes soon to publish ; and the proper 

 motions contained in it are rendered more trustworthy by the 

 enlightenment of new observations. The positions of the stars 

 are all brought up to the epoch 1900-0. 



GEOGRAPHICAL NOTES. 



M. Charles Allnand describes his researches on the 

 Island of Mahe, the largest of the Seychelles Group (see Nature, 

 p. 162), in a letter to the Paris Geographical Society. He has 

 studied the fauna with some care, and remarks on the singular 

 poverty of animal life compared with the great luxuriance of 

 vegetation. In Port Victoria, the chief settlement in Mahe, the 

 only form of butcher-meat obtainable is the flesh of the great 

 turtle [Chelone midas) whose shell is valueless, the tortoise-shell 

 fisheries of the island depending on the Chelone imbricata. M. 

 Allnand hopes to bring back with him living specimens of the 

 elephantine turtles of the Aldabra Islands, specimens of which 

 have been transported to the Seychelles. 



The expectation of an Antarctic expedition, on which valu- 

 able scientific observations might have been made, has proved 

 illusory. Captain Gray, of Peterhead, had organized a whaling 

 voyage to the far south, and appealed to the public for 

 funds to carry it out with some prospect of commercial success, 

 but the response was so unsatisfactory that the enterprise has 

 been abandoned. From a scientific point of view, the advantages 

 of Antarctic exploration is so great, and the probability of valu- 

 able practical results to apparent, that the apathy alike of the 

 British and Australian Governments as well as of the general 

 geographical public is incomprehensible. The fact that no 

 steamer has ever been despatched to the south of the Antarctic 

 Circle with the object of attaining high latitudes says much for 

 the prudence and little for the energy of present-day explorers. 



A Chair of Colonial Geography is about to be established 

 at the Sorbonne for the special study of the French colonies. 



The discovery of America by Columbus is to be celebrated in 

 Hamburg on October 11 and 12 by gatherings of delegates from 

 the German Universities and Geographical Societies, by whom 

 papers bearing on German enterprise in the sixteenth century 

 will be read. An exhibition of articles illustrating the early 

 connection of Hamburg and America will also be held. 



The Manchester Geographical Society has just published its 

 Journal for July-September 1891, containing several intcrest- 

 mg papers on India and a variety of short notices. It is unfor- 

 tunate that the small local encouragement given to this Society 

 makes the earlier publication of its memoirs possible. Surely 

 Manchester could afford and should endeavour to maintain a 

 Geographical Society as prosperous financially as it is enterpris- 

 ing and persevering. The contrast between the many provin- 

 cial Geographical Societies in Germany and France with the 

 three already established in England corresponds to the relative 

 interest in geography as an aid to commerce on the Continent 

 and in Great Britain. 



METALLIC CARBONYLSy 



JUSTUS LIEBIG, perhaps the most prophetic mind among 

 modern men of science, wrote in the year 1834 in the 

 Annalen der Pharniacie. " I have previously announced that 

 carbonic oxide may be considered as a radical, of which carbonic 

 acid and oxalic acid aie the oxides, and phosgene gas is the 

 chloride. The further pursuit of this idea has led me to the 

 most singular and the most remarkable results." 



Liebig has not told us what thtse results were, and it has 

 taken many years before the j-rogress of chemical research has 

 revealed to us what may at that early date have been before 

 Liebig's vision. I will to-night bring before you some important 

 ' Friday Evening Discourse delivered at the Royal Institution by Ludwig 

 Mond, t.R.S , on Junes. 



