Mays, ^^77] 



NA TURE 



15 



in Nature last week, the council of the Royal Astronomical 

 Society have guaranteed 500/. for the expenses of Mr. Gill's 

 expedition. Ascension has been fixed upon, not without a 

 careful consideration of probable meteorological conditions about 

 the time of the opposition of Mars in September, in which it is 

 understood the records of the Meteorological Office have been ol 

 the greatest service, and in fact, have induced Mr. Gill to fix 

 upon Ascension for the site of his temporary observatory in pre- 

 ference to St. Helena, the astronomical condition being about 

 the same for the two islands, i.e., their latitudes not differing much 

 from the declination of the planet when nearest to the earth, so 

 that it is observable at a considerable hou -angle on both sides 

 of the meridian. 



The modus operandi ■^\o\so%<:A by Mr. Gill, is as follows : — two 

 stars, a and b, one preceding and the other following the planet, 

 are selected for each night of observation, and their angle of 

 position and distance relative to the planet are computed roughly 

 for 4h. E. and 4h. W. hour-angle, and the right ascension and 

 declination of the middle point between siar and planet ; so that 

 the proper stars of comparison are readily found. The helio- 

 raeter axis is directed to this middle point, the position-circle 

 set to the position angle, and the segments set to the approxi- 

 mate distance. The observer finds in the field of view a star 

 and the planet ; by turning the handle by which the segments are 

 moved in distance, the images of star and planet are made to 

 move relatively to each other in the direction of a line joining 

 the objects, while if the position-handle is turned, the images 

 move in the direction of a line perpendicular thereto. Suppose 

 that the star a is viewed through segment A, and the planet 

 through segment B. According to Mr. Gill's usual practice the 

 observation would proceed thus : — 



I. Measure of position-angle. 

 II. Measure of distance, both limbs. 

 III. Reverse segments, and view star hy segment B and 



planet by segment \. 

 IV. Measure position-angle. 



This constitutes one measure. 

 V. Repeat this process with star /'. 



VI. Reverse position-circle and repeat the co nparison with 

 star b. 



VII. Compare again with star a. 



This constitutes a complete symmetrical set, which Mr. Gill 

 has found can be secured on an average in ih. 30m., sometimes 

 in ih. lom., or if there be interference from cloud it may 

 occupy 2h. 



In the measure of a position-angle, by a movement of the 

 handle for distance, the star may be made to move, relatively to 

 the planet along the line of separation of the lenses, so that the 

 star successively occupies positions I, 2, 3 . . . 3, 2, i, &c. 

 This motion may be very slow and the position-circle being set so 

 that the motion of the planet completes the bisection, the 

 observer has only to go on moving the star slowly till the limb is 

 seen to symmetrically bisect the star (the time of which is noted) 

 precisely as Jupiter's limb bisects one of his satellites. 



The measure of distance is conducted with equal care, but is 

 not so readily explained without a diagram. Mr. Gill finds his 

 method possesses very great delicacy. It sometimes happens 

 that it is not possible to find a star sufficiently bright to compare 

 with Mars in his full light. In such cases the brilliancy of the 

 planet can be easily kept down by a wire-gauze screen, which, 

 by an arrangement at the eye-end, can be laid over either seg- 

 ment of the object-glass and at any angle thereto. 



In a letter to M. Leverrier, published in the Bulletin Interna- 

 tional of April 27, Mr. Gill states that the observations of Juno, 

 which he made with Lord Lindsay at the Mauritius with the 

 same heliometer, showed that the determination of the diurnal 

 parallax by measuring with this instrument the distance of the 

 planet from a star preceding and a star following is susceptible 



of an extreme precision, and he found the probable error in the 

 determination of the planet's position for each complete observa- 

 tion of the morning or evening did not exceed -J- o"'075. Lord 

 Lindsay has slated that the value of the solai parallax, resulting 

 from these observations of Juno (a single discordant one only 

 being rejected) is 8"S2, which approaches near to Prof. New- 

 comb's value, S"SS, adopted provisionally by the German 

 astronomers, and to M. Leverrier's theoretic?.! determination, 

 8" '86. This sufficiently indicates the utility of the method, and 

 Mr. Gill intends to avail himself of the close oppositions of the 

 minor planets Ariadne and Melpomene during his visit to Ascen- 

 sion to obtain values of the parallax by observation on the same 

 principle. 



Court 1S77 II. (VVinnecke, April 5). — This comet may 

 be expected to prove a fine telescopic object during the absence 

 of moonlight in the circumpolar sky, with its stellar-looking 

 nucleus and double or broad fan-shaped tail. The annexed posi- 

 tions for midnight at Berlin are from elements by Herr Plath, of 

 Hamburg, and have been received from Prof. Winnecke : — ■ 



■R 4 r)prlln-.t;nn ^^S- distance 



^■■^ Ueclmation. from Earlli., 



fi. m. s. „ , 



-f 65 59-2 ... 9-99601 



6S 7 '2 ... 9-99640 



70 u-6 ... 9-99741 



72 1 1-3 ... 9-99902 



74 4-5 ... 0-COI23 



75 488 ... 0-00400 



77 21-5 ... 0-00732 



78 38-3 ... 0-01116 



79 3S'i ■■■ o'0>549 



80 7-S ... 0-02028 

 •I- 80 13-6 ... 0-02548 



The following orbit has been calculated by Mr. Hind from 

 observations at Strasburg, on April 5 and 25, and at Berlin and 

 Lfiipsic on April 14: — 



Perihelion Passage, 1877, April 17-646S7, G.M,T. 



Longitude of Perihelion 253 30 9 ) Mean Equinox, 



Ascending Node ... 316 33 53 i 1877-0 



Inclination 58 54 22 



Distance in Perihelion 0-950250 



Heliocentric motion — retrograde. 



These elements represent the observations during the interval 

 very closely. 



May 4 ... 23 26 5 



5 ■■■ 23 36 41 



6 ... 23 49 22 



7 ■•• 04 45 



8 ... o 23 31 

 y ... o 46 49 



10 ... I 15 34 



11 ... I 50 47 



12 ... 2 32 59 



13 •■■ 3 19 55 



14 ... 4 S 57 



NOTES 



M. Fl.\mmarion has been authorised by M. Leverrier to use 

 one of the largest refractors of the Paris Observatory for the in- 

 vestigation of the motion of double-stars round a common centre 

 of attraction. This liberality on the part of the chief of the 

 Paris Observatory is highly creditable. M. Leverrier, indeed, ii 

 desirous of placing the immense means of investigation pos- 

 sessed by the observatory at the service of a number of inde- 

 pendent workers not belonging to the staff of the establishment, 

 but who have given solid proofs of their zeal and capacity for 

 research in some particular science. His ambition is to create at 

 the observatory a national astronomical institution where quali- 

 fied scientific men may find ample means for following their own 

 special studies. 



Sir D.wiD Monro, late Speaker of the House of Representa- 

 tives in New Zealand and an active promoter of science in that 

 colony, died at Nelson, New Zealand, on February 15. He 

 graduated in Medicine in the University of Edinburgh in 1834, 

 where his great grandfather, grandfather, and father successively 

 held the Chair of Anatomy. He devoted the leisure of an active 

 political life I0 the pursuit of botany, and by his discoveries, 

 which were published by Dr. Hooker in his "New Zealand 



