May II, 1876] 



NATURE 



29 



Spring Dynamometers 



In a former brief communication of mine on the subject of 

 dynamometers (Nature, vol. xiii. p. 385), suggested by an in- 

 cidental remark made by Mr. Bottomley, I observed that "about 

 three years ago Prof. Ball when introducing the C. G. S. system 

 of units into the course of mechanics in this College had a series 

 of dynamometers in absolute measure specially constructed for 

 him." In reference to this statement, Dr. Ball's successor in 

 the chair of mechanics, Prof. Ilennessy, points out, in a letter 

 to NiVTURE (vol. xiii., p. 466), that "the system actually 

 employed is not that referred to by your correspondent ; I 

 generally employ the kilogram, metre, and second, and some- 

 times the foot, pound, and second, to measure a dynam or unit 

 of force." It is, however, evident that the few words in my 

 former letter did not question the merits of any particular system 

 of units ; whether the use of a mixed system of kilogram-metres 

 and foot-pounds be an improvement upon a system now generally 

 coming into use is a matter of opinion. And though the sub- 

 ject can hardly be one of much interest to your readers, I 

 may, perhaps, remark that so far as my statement concerns Dr. 

 Ball it is perfectly accurate ; he was in the habit of using the 

 C. G. S. system in his classes here, and I was unaware any change 

 had been made in this respect, the following statement occurring 

 in Prof. Hennessy's own syllabus for the present as well as last 

 session : — "The unit of force employed is the 'dyne,' or that 

 force which, acting uniformly upon one gramme for one second, 

 will give it a velocity of one centimetre a second." Even if 

 reference had been made to Prof Hennessy, one would naturally 

 have concluded that the printed syllabus, authorised by the 

 Department, was the one "actually employed." 



Passing on to Dr. Ball's dynamometers. Prof. Hennessy re- 

 marks that " they cannot be depended upon to results within the 

 tenth of a kilogramme " — finer readings when necessary could, no 

 doubt, be taken by the eye, but that is really only a question 

 for the maker, and the special purpose for which these instru- 

 ments were designed : then follows the strong assertion that 

 " spring dynamometers are totally unfit for measuring units on 

 the C. G. S. system." As several instruments of precision 

 depending on the tension of a spring recur to one's mind, instru- 

 ments that only require proper precautions to yield extremely 

 delicate and trustworthy results, it would be interesting to know 

 upon what grounds Prof. Hennessy bases his emphatic and 

 reiterated assertion. If it be merely a question of individual 

 opinion, upon this subject hardly any authorities that could be 

 quoted would carry such weight as Sir W. Thomson and Prof. 

 P. G. Tait, who speak thus in their treatise on "Natural 

 Philosophy," p. 127, "Spring balances we believe to be 

 capable, if carefully constructed, of rivalling the ordinary balance 

 in accuracy, while for some applications they far surpass it in 

 sensibility and convenience." 



Royal College of Science, Dublin W. F. Barrett 



The Meteors of April 20th 



Between ten and twelva o'clock on the night of April i8th, 

 Mr. W. L. Taylor, a member of the junior class in the State 

 University, with several other gentlemen, observed an unusual 

 number of shooting-stars. These gentlemen were returning in 

 an open waggon from Ellettsville, eight miles north of Bloom- 

 ington. No count was kept of the number of meteors observed, 

 but the appearance was so frequent as to attract the attention of 

 all the company. Mr. Taylor thinks the number noticed could 

 not have been less than twelve or fifteen. From the descriptions 

 given of the meteor tracks, I find that they were nearly conform- 

 able to the radiant of the Lyraids. The meteors were remark- 

 ably brilliant, apparently equal to stars of the first or second 

 magnitude. 



At my request Mr. Benjamin Vail, a student of the University, 

 made observations on the nights of the 19th and 20th of AprU. 

 Both nights were so cloudy, however, that a continuous watch 

 would have been useless. About eleven o'clock on the night of 

 the 19th, three meteors were seen in the north-west, where the 

 sky at the time was partially clear. 



Bloomington, Ind., April 26 Dahiel Kirkwood 



American Mocking Bird 



An American mocking-bird, about a year old, which I had 

 brought from Tennessee, has, for the past three or four weeks, 

 been affected with an irritation round the eyes, causing the 



feathers to fall off and the flesh to swell ; the bird is otherwise 

 in a healthy condition, but has not sung since it has been affected 

 with the soreness ; it has the proper food supplied, and its cage 

 is kept in a clean state ; could any correspondent kindly inform 

 me the cause and cure of the disease ? M. C. 



An Unusual Optical Phenomenon 

 This morning, a little after nine o'clock, the ordinary solar 

 halo, radius about 22°, was seen. It was bright, and the red 

 very distinct. 



On turning to the north to find the direction of the cloud drift, 

 a white band was seen extending to the north-east in one direc- 

 tion, and on to the west and south in the other. Its width was 

 about that of the halo near the sun. A pau: of compasses and a 

 protractor gave the altitude of this circle about 45". This 

 being about the sun's altitude, the plane of the circle was no 

 doubt parallel to the horizon and passed through the sun. I 

 believe the circle above described to be but rarely seen. 



Joseph Gledhill 

 Mr. Crossley's Observatory, Halifax, May 3 



OUR ASTRONOMICAL COLUMN 



The Binary X Ophiuchi.— An examination of the re- 

 cent measures of this star, shows that neither of the orbits 

 computed some 25 or more years since by Madler and 

 Hind at all represents the later course of the companion, a 

 circumstance mainly attributable, as it appears, to error 

 in one, if not in both, of Sir W. Herschel's measures. 

 Struve at first considered that the angle of 1 783 required 

 a correction of 180°, but at a later period he was inclined 

 to apply a similar coiTection to the angle of 1802, and 

 Dawes also believed it was the latter measure which re- 

 quired alteration, in order to render any orbit possible. 

 It is upon this supposition that the orbits of Madler and 

 Hind have been calculated : the two sets of elements are 

 subjoined : — 



Ma.Uer. Hind. 



Peri-astron passage 179031 1791*21 



Period of revolution in years 8901 95 "88 



Node 32"^ 42' 30° 23' 



Angle between the lines of nodes ) go , ^ / 



and apsides on orbit \ ^ '^S 24 



Inclination 49° 25' 49° 40' 



Excentricity 0*4530 0*4772 



Semi-axis major ... ©"842 o"'847 



Midler's orbit was published in " Untersuchungen iiber 

 dieFixsterne-Systeme, Erster Theil." The second orbit was 

 founded upon observations to about the same year, 1849. 

 The projection of the measures since this epoch, however, 

 makes it apparent that the real orbit must be materially 

 different from the above, and the star may be recom- 

 mended to the attention of those who are interested m 

 the determination of elements of the revolving double- 

 stars. 



Sir W. Herschel's papers containing his nseasures of 

 double stars communicated to the Royal Society, not 

 being always of easy access, the following extracts from 

 his notes on \ Ophiuchi may perhaps prove useful : — 



From the PhiL Trans., vol. Ixxv., p. 62 : — 



"I. 83; 1783, March 9. A very beautiful and close 

 double-star, L. w. ; S. blue ; both fine colours. Con- 

 siderably or almost very unequal. With 460, J- or ^ 

 diameter of S. ; with 932 full \ diameter of S. Position 

 14° 30' n. following." 



From the memoir of 1804 — 



" May 20, 1802, position was 20° 41'. The position 

 March 9, 1783, was 14° 30', north following. The difference 

 in nineteen years and seventy-two days is 6" 11'. May i 

 aod 2, 1 802, I could not perceive the small star, though 

 the last of the two evenings was very fine. May 20, 1802, 

 with 527, I saw it very well, but with great difficulty. The 

 object is uncommonly beautiful, but it requires a most 

 excellent telescope to see it well and the focus ought to 



