2l8 



NATURE 



[December 15, 1910 



-chamber, which could be evacuated and dried. The 

 observations were made by tilting a large plate of the 

 material with a perfectly smooth surface until a small 

 piece of the same substance, provided with three spherical 

 feet, would slide down the surface. Sliding begins at a 

 very small angle, 1-5° for glass, and the speed has a 

 fixed terminal value for each angle of tilt up to a little 

 more than 5", when the motion becomes an accelerated 

 one. Up to this point there is a definite relation between 

 the speed and the friction, and this relation must be 

 substituted for the discontinuous law of Coulomb, accord- 

 ing to which friction prevents motion until an angle of 

 tilt of the order of 10° or 20° is attained. 



An interesting article on critical speeds for torsional and 

 longitudinal vibrations, by Prof. Arthur Morley, of 

 University College, Nottingham, appears in Engineering 

 i'or December 9. The driving effort of a reciprocating 

 engine, or the resistances to be overcome, may be periodic- 

 ally fluctuating in magnitude, and if the period of such a 

 variation should approach to the period of a free torsional 

 vibration, or to an integral multiple of it, torsional oscilla- 

 tions of some considerable magnitude may be set up, with 

 accompanying high stresses in the material of the shaft. 

 Cases of approach to dangerous resonance with longitudinal 

 vibrations are perhaps much less common in machinery. 

 The author gives a complete mathematical analysis in the 

 article, and concludes with an interesting application to the 

 case of a pit cage and contents weighing ten tons, and 

 being raised by an engine running at 100 revolutions per 

 minute. Taking the net section of the rope at 2-5 square 

 inches and E as 13,000 tons per square inch, the depth at 

 which the natural frequency of vibration of the loaded rope 

 is equal to the speed of the engine is 955 feet, neglecting 

 the weight of the rope. Taking a rope weighing 8-4 lb. 

 per foot, and making allowance for its weight, the depth 

 at which resonance will occur works out to about 862 feet. 



OUR ASTRONOMICAL COLUMN. 



Nova Ar^, 98.1910. — A telegram from Dr. Ristenpart 

 to the Astronomische Nachrichten (No. 4457) states that 

 the magnitude of Nova Arae, recently discovered by Mrs. 

 Fleming, was 9-6 on November 19. 



The nova is invisible on forty-four plates of the region 

 taken at Arequipa during the period August, 18S9, to 

 March 19, 19 10, but appears on twenty-one photographs 

 secured between .April 4 and .August 3 of this year ; on 

 these plates its magnitude apparently ranges from 6-o to 

 lo-o, and thus it would appear that between March 19 

 and -April 4 the magnitude increased from 12-0, the limit- 

 ing magnitude of the .Arequipa plates, to 6-o. Like most 

 of its class, this nova lies well in the Milky Way, its 

 position (1875-0) being a=:i6h. 31m. 4s., 5=— 52° 10-4'. 



Saturn's Rings.— Herr K. Schiller, writing to the 

 Astronomische Nachrichten (No. 4458), states that he 

 observed Saturn's ring system on November 26 at Both- 

 kamp, and could detect no extraordinary feature such as 

 was described by M. Jonckheere in an earlier communica- 

 tion ; the atmospheric conditions were excellent, and Herr 

 Schiller employed powers of 200, 600, and 800. 



Publications of the Allegheny Observatory. — We 

 have received the first four numbers of vol. ii. of the 

 Publications of the Allegheny Observatory of the Uni- 

 versity of Pittsburgh, and give brief abstracts of them 

 "below. In No. i Prof. Schlesinger describes the Mellon 

 spectrograph with which he and the other observers prose- 

 cute their radial-velocity researches. This instrument was 

 provided, by the generosity of Mr. Andrew Mellon, for 

 iine-of-sight work when the Keeler memorial telescope 

 ■was completed in 1906. The grave disadvantages arising 

 from the location of an astronomical observatory near a 



NO. 2146, VOL. 85] 



large town, where the sky is never clear and ever 

 illuminated by artificial illuminants, had to be considered 

 when the form of instrument was under contemplation. 

 Consequently, the work which is possible had to be 

 materially restricted, because of the necessity of keeping 

 the possible exposures within reasonable limits, and a one- 

 prism spectrograph was designed. The sacrifice was cone 

 siderable, but peculiar circumstances rendered it necessary. 

 However, it appeared that useful work might be done if 

 the investigations were confined to such stars as have 

 broad, hazy lines, and this decision has been justified by 

 the results already published. Dr. Schlesinger describes 

 and illustrates the details of the instrument, showing how 

 rigidity has been attained with moderate weight. A region 

 of the spectrum from A 3930 to A. 4750 can be brought 

 into sharp focus, and under exceptionally good conditions 

 a strong spectrum of a fifth-magnitude star can be obtained 

 in about twenty minutes. Owing to the impurity of the 

 town atmosphere, the large mirror of the Cassegrain re- 

 flector has to be resilvered once a month, and the small 

 one everv other week ; even then, at times, they only 

 reflect about half as much light as when newly silvered, 

 and some 40 per cent, of the deterioration takes place 

 within three or four days of resilvering. The arrange- 

 ments for maintaining the temperature range of the prism 

 box within 01° C, for eliminating flexure, and for adjust- 

 ing the focus are minutely described and very ingenious. 



In No. 2 Dr. Schlesinger and Mr. D. .Alter discuss the 

 relati%'e motions of 61 Cygni and similar stars. This dis- 

 cussion indicates that the motion of the companion star 

 is orbital rather than in a straight line — that the two stars 

 are phvsically connected ; thus the designation " of the 

 61 Cvgni type," as indicating pairs not physically con- 

 nected, should be abandoned. 



No. 3 contains a discussion of the orbits of the spectro- 

 scopic components of € Herculis, by Dr. R. H. Baker, 

 determined from seventy-two plates taken with the Mellon 

 spectrograph during 1907-8. The period is found to be 

 4-0235 days, and the orbit nearly circular; there is no 

 substantial evidence for the presence of a third body. In 

 No. 4 Dr. Baker discusses the orbit of i H. Cassiopeia?, 

 from fifty-seven plates secured during 1908-9, and finds the 

 period to be 6-067 days. 



The Orbit of the Perseids.^ — Meteoric astronomy is 

 being, and is likely to be, considerably advanced by the 

 energetic and organised observations of the Antwerp 

 Society d '.Astronomic. Since 1896 the Perseid and other 

 showers have been independently observed at many 

 stations, and the results collated and discussed. During 

 1909 and 1910, 485 and 303 Perseid trails were recorded, 

 and indicate the existence of seven radiants. For five of 

 the best marked of these M. Henri Dierckx has calcu- 

 lated elliptic elements, which he compares with Hayn's 

 elements for Tuttle's comet, 1862 III., in an article appear- 

 ing in Nos. 11-12 of the Gazette astronomique. The 

 agreement is well marked, although, as the author re- 

 marks, the probably large area covered by the swarm of 

 meteoritic particles precludes the expectation that the 

 Perseid elements would rigidly agree inter se. 



Definitive Elements for the Orbit of Comet 1904 II. 

 (1904^). — This comet was discovered by.M. Giacobini at 

 Nice on November 17, 1904, and was observed until May 

 2, 1905. Herr J. Sedlacek has now discussed 118 observa- 

 tions, referred to eighty-four comparison stars, and 

 publishes the resulting orbital elements in No. 4453 of the 

 Astronomische Nachrichten. The orbit appears to be 

 hyperbolic, but the departure from a parabola is so slight 

 as to be practically negligible. 



Designations of Newly Discovered Variable Sx.'iRS. 

 In No. 4457 of the Astronomische Nachrichten the com- 

 mission of the AG Catalogue of Variable Stars gives the 

 permanent designations to 126 recently discovered variable 

 stars. Manv of the objects have been discovered to be 

 variable during the present year, whilst the variabilitv of 

 others was detected during preceding years. In addition 

 to the designations, the commission gives the provisional 

 numbers, the positions for 1900, the range of magnitude, 

 and remarks concerning the discovery, the period, the type, 

 and the spectrum of each object. 



