472 



NATURE 



[August 9, 19 17 



to photographic recording. A spot of light from a 

 slit or hole is allowed to fall on to the mirror of a 

 mirror-galvanometer, whence it is reflected on to a 

 sensitised plate. This method is frequently used 

 nowadays in investigating the critical points of steel. 

 The writer mentions the recording apparatus thought 

 out iby M. Saladin, which permits of tracing all kinds 

 of curves on a photographic plate by using two 

 mirrors both capable of rotation aibout a vertical axis 

 and operated by two galvanometers. A fixed mirror, 

 inclined 45° to the horizontal, is placed between the 

 two moving mirrors. The ray of light reflected by 

 the first mirror is thus given a horizontal movement, 

 which becomes vertical after reflection by the fixed 

 mirror at 45°. It is then reflected on to the second 

 mirror, which imparts to it a second and final hori- 

 zontal displacement. Finally, the combination of 

 these two perpendicular movements traces a curve on 

 a stationary photographic plate. It is thus possible 

 to plot directly curves of electric resistance, of dila- 

 tation, and of E.M.F. in terms of temperature. 



In view of the importance of internal waterways 

 and of the many questions which revolve round the 

 development of the hydraulic resources of their country, 

 the National Association of Navigation Congresses in 

 Italy has inaugurated the publication of a small period- 

 ical of eight pages, known as Navigazione Interna, 

 to deal with matters of interest in that -connection as 

 they arise. The first issue, for May, 1917, lies before 

 us and contains an account of the work of the Hydro- 

 technical Institute at Stra, associated with the Uni- 

 versity of Padua and the Hydrographic Department 

 at Venice, describing in particular the experimental 

 tank 200 metres (656 ft.) long, with a (bottom width 

 of 370 metres (12 ft.), a top width of 1075 metres 

 (354 ft.), and a mean depth of 350 metres (115 ft.). 

 The tank is constructed in cement concrete, with light 

 metal reinforcement, and is fitted with a moving plat- 

 form arid the appropriate mechanical equipment. The 

 institute undertakes experimental work in connection 

 with aH hydraulic problems, including those in regard 

 to the resistance offered to the movement of solid 

 bodies in water. Another interesting article in the 

 journal deals with a Swiss project of a navigational 

 connection between Lake Maggiore and the River Po. 

 We welcome the advent of this latest recruit to the 

 service of hydrological science. 



Evidence of the dilficultv of destroying reinforced- 

 concrete buildings is given in an article on concrete 

 in war in the Times Engineering Supplement for July 

 27. Steel cupolas have been blown to fragments by 

 high-explosive shells, while similar structures in re- 

 inforced concrete have survived the ordeal with com- 

 paratively little injury. Many of the reinforced con- 

 crete buildings plentifully scattered over Northern 

 France have been used by the Germans as well as by 

 ourselves, and have shown remarkable capacity for 

 withstanding artillery fire. One case quoted is of an 

 elevated reservoir measuring about 80 ft. long, 40 ft. 

 wide, and 12 ft. deep, supported on a framework ot 

 thin columns more than 40 ft. high, with horizontal 

 bracings. The flat roof at a height of 55 ft. was used 

 by the Germans as a ready-made observation post. 

 This reservoir, built in June, 191 1, of Hennebique 

 ferro-concrete, was destroyed so far as possible when 

 the Germans evacuated the town in March last, having 

 successfully withstood our bombardment, which de- 

 stroyed all surrounding buildings. The columns were 

 broken by explosives, allowing the reservoir proper to 

 fall to the ground, where it remained intact save for a 

 few cracks and holes cut in the corners, where explo- 

 sives had been inserted with the object of tr\'ing to 

 damage the walls. 



NO. 2493, VOL. 99] 



OUR ASTRONOMICAL COLUMN. 



Discovery of a New Star. — According to the 

 Times of August 6, the discovery of a new star by 

 Mr. Ritchey on July 19 has been announced by Prof. 

 Pickering. The star is very faint, being of the 14th 

 magnitude, but it may be of special interest because of 

 its situation in the nebula N.G.C. 6946. This is a 

 large faint nebula in Cygnus, its position being R.A. 

 2oh. 33m. 17s., decl. +59° 51'. 



Stellar Motions and Absolute Magnitudes. — The 

 relationship of stellar motions to absolute mag- 

 nitudes has been further investigated, as re- 

 gards 1300 stars of types F, G, K, and 

 M, by W. S. Adams and G. Stromberg {Astro- 

 physical Journal, vol. xlv., p. 293). Parallaxes deter- 

 mined by the spectroscopic method were available for 

 about 700 of the stars used, and these were supple- 

 mented by others derived with the aid of a new 

 formula connecting mean parallax with proper motion. 

 From the parallaxes and apparent magnitudes, the 

 absolute magnitudes are easily calculated, being de- 

 fined as apparent magnitudes reduced to the distance 

 corresponding to a parallax of o-i". The stars being 

 divided into groups defined by certain limits of 

 parallax, it is first shown that among stars at the same 

 distance from the sun there is an increase of radial 

 velocity with decrease in absolute brightness, and that 

 there is little evidence of any variation in radial velo- 

 city depending upon distance from the sun. For the 

 spectral types considered, the increase in velocity is 

 1-5 km. for a decrease in brightness of one magnitude. 

 It is shown that this effect cannot be ascribed either to 

 distance from the sun, to the law of frequency-distri- 

 bution of the velocities, or to the eftect of stream 

 motion. The same conclusion is reached with regard 

 to the cross linear motions of the stars, and it appears 

 to hold in the mean for apparent as well as for abso- 

 lute brightness. The stars of types K and M have 

 mean velocities about i-o to 1-5 km. higher than the 

 F and G stars of the same absolute magnitude. 



Union Observatory, Johannesburg. — Circular No. 

 37 of the Union Observatory includes measures of 

 ninety-eight double stars and the results recently ob- 

 tained by the blink microscope in connection with 

 proper motions and new variable stars. On twenty- 

 three pairs of plates taken at the Melbourne Observa- 

 tory, at intervals averaging about twenty years, 104 

 proper motions were found and measured, twenty- 

 three of the stars having a centennial proper motion 

 of 20" or more ; fifty-six of these stars have a motion 

 nearly parallel to the galactic plane and towards the 

 solar ant-apex. Proper motions amounting to only 

 004'' or 006* a year were easily and certainly deter- 

 mined. A disagreement with the results from astro- 

 graphic measurements in the case of three pairs of 

 plates taken at the Cape Observatory has led to an 

 interesting correspondence with Prof. Kapteyn. The 

 general impression obtained by Mr. Innes from his 

 work with the blink microscope is that practically all 

 stars, nebulae, and clusters in any one region are at 

 the same distance from us, and that with very few 

 exceptions they are all relatively fixed. In Circular 

 No. 38 Mr. Innes announces an important undertaking 

 in the form of a photographic map of the southen. 

 sky, from the Franklin-Adams star plates. Each 

 chart will be ruled with hour circles and parallels, 

 and will cover a little more than 30 square degrees, 

 the scale being 36 mm. to 1°. The region to be 

 covered will require 556 charts. An excellent speci- 

 men chart accompanies the circular, and others will 

 be issued as opportunity offers. 



