September i6, 1909I 



NATURE 



355 



dangerous constituent, leading to fractures as well as to 

 corrosion. 



Prof. O. Lehmann, of Carlsruhe, who is so well known 

 for his work on liquid crystals, has done a great service 

 to those wishing to repeat any of the beautiful experi- 

 ments which can be performed with these bodies by 

 giving, in the Physikalisclie Zeitschrift for August 15, de- 

 tailed descriptions of thirty-two of the most convenient 

 and suitable experiments to perform during a lecture on 

 the subject. 



The Physical Review for July contains the second of a 

 series of communications from Mr. G. W. Pierce, of the 

 Jefferson Ph)'sical Laboratory of Harvard, on the behaviour 

 of rectifiers of alternating electric currents such as are used 

 as detectors of electric waves. In the present case electro- 

 lytic rectifiers have been studied by the aid of the Braun 

 tube oscillograph, and the author finds that the theory of 

 electrolytic polarisation is capable of explaining all the 

 facts observed, if the slight polarisation capacity of the 

 small platinum electrode of the rectifier is taken into 

 account. The detector, when polarised by the superposition 

 of a direct current, is almost perfect, that is, the current 

 it produces is nearly all in one direction. It may there- 

 fore be compared with the crystal rectifiers dealt with in 

 the author's first paper. The author proposes to. examine 

 the behaviour of vacuum-tube rectifiers before giving 

 definite shape to any theory of crystal rectifiers. 



The Century Magazine for September contains two 

 interesting engineering articles. The first of these deals 

 with the great aqueduct now being constructed for bring- 

 ing water from the Catskill Mountains to the City of New 

 York. This aqueduct will be ninety-two miles long, and, 

 to supply the 500,000,000 gallons required daily, more than 

 600 square miles of collection area must be utilised and 

 several large reservoirs constructed. The article is well 

 illustrated with photographs, sections, and maps. The 

 second article is a first instalment giving an account of 

 Fulton's invention of the steam-boat. While many 

 engineers in this and other countries experimented towards 

 the end of the eighteenth century, Fulton was the first 

 to secure real success. However, it may comfort some 

 of our British patriots to be reminded that the American 

 vessel was fitted with one of Watt's engines, constructed 

 .in Birmingham and shipped to America. The article con- 

 tains many original documents and drawings, and is of 

 interest as showing that the modern troubles which many 

 inventors have to face in worlcing out their schemes and 

 in overcoming red tape had their counterpart more than 

 a century ago. 



The results of some experiments on solid steel bars 

 under combined stress are given in an article in Engineer- 

 ing for August 20. The author, Mr. C. A. Smith, of the 

 East London College, University of London, has already 

 presented useful work in confirmation of Guest's law, and 

 in this series has loaded solid specimens in compression 

 and torsion, and also in tension and torsion. The necessity 

 for doing this will be evident when it is remembered that 

 Lord Kelvin suggested that, as a tension load lowers the 

 torsional yield point, a compression load would raise it. 

 Four different grades of steel were experimented on, of 

 carbon content ranging from 009 per cent, to 0-48 per cent. 

 Under combined compression and torsion, the maximum 

 principal stress at yielding varied from 19,800 lb. to 

 36,500 lb. per square inch, while the maximum shear stress 

 varied from 18,900 lb. to 20,400 lb. per square inch, the 

 average variation from the mean of the latter stress being 

 2.i6 per cent. Another series shows an average variation 

 NO. 2081, VOL, 81] 



of the maximum shear stress from the mean of 1-87 per 

 cent, only, this series including tests in tension, com- 

 pression, torsion, and combined stress. Taking all the 

 results given, the average variation from the mean of the 

 maximum shear stress is about 2 per cent. The import- 

 ance of these tests will be understood when the difficulties 

 of testing solid specimens under combined stress are re- 

 membered, difliiculties which seem to have been overcome 

 successfully by use of the author's sphingometer, by means 

 of which the tension and compression measurements are 

 taken in three planes. 



Referring to the letters published in Nature of July 22 

 and 29 in regard to sonorous or musical sands. Prof. J. C. 

 Branner, Stanford University, California, writes to direct 

 attention to articles on this subject by Profs. H. C. Bolton 

 and Alexis A. Julien, published in the Proceedings of the 

 American ' Association for the Advancement of Science 

 (vol. x.Kxii., pp. 251-2 ; vol. xxxiii., pp. 408-13 ; vol. 

 xxxviii., pp. 137-40). We may remind Prof. Branner that 

 the subject was discussed in Nature by Prof. Bolton and 

 Mr. Carus-Wilson twenty years ago (vols, xxxix.-xlvi.). 



A NEW edition of Mr. P. H. L'Estrange's " Junior 

 Course of Comparative Geography " has been published 

 by Messrs. George Philip and Son, Ltd. Part v. of this 

 book, too, has now been issued separately at the price of 

 lod. In the new edition all the maps of the original 

 work have been reproduced in black and white, the names 

 and symbols required for this course only being retained. 

 The book has been revised throughout, and additional 

 matter added, for example, on local geography. 



OUR ASTRONOMICAL COLUMN. 



Hallev's Comet Re-discovered. — To Prol.. Ma.x Wolf 

 belongs the honour of re-discovering Halley's comet after 

 an absence of more than seventy years. A telegram from 

 the Kiel Centralstelle announces that the comet was dis- 

 covered at the Kbnigstuhl Observatory on September 11. 

 Its position at I4h. 7-3m. (Konigstuhl M.T.) was 

 R.A. = 6h. i8m. 12s., dec. = + 17° 11', 



and its magnitude 160. 



Mr. Crommelin's ephemeris position for September 11-9 

 was 



R..\. = 6h. i8m. 4s., dec. 4-17" 16', 



so to the Greenwich calculators, Messrs. Cowell and 

 Crommelin, must be given the credit of having prepared 

 an ephemeris which agrees remarkably well with the 

 observation. At present the comet is approaching the 

 northern limit of Orion from the south-western region of 

 Gemini, forming nearly a straight line with tlie stars 

 7 Geminorum and 143 02, the three objects being about 

 equally spaced in the order 7 — 143 02— comet. The 

 following is an extract from Mr. Crommelin's ephemeris : — 

 September 25-7, 6h. i8-5m., 4-17" 11'; October 9-1, 

 6h. i4-6m., 4-17° 8'; October 22-0, 6h. 4-9m., 4-17° 2'. 



Observations of Mars. — Some interesting observations 

 of changes, during August, in the areas surrounding^ the 

 southern ice-cap of Mars are reported by MM. Antoniadi, 

 QuiSnisset, and Jarry-Desloges in the September number 

 of the Bulletin de la Societe astronomique de France 

 (pp. 385-94). M. .\ntoniadi, observing at Juvisy on August 

 12, 14, and 16, found the planetary features so pale as to 

 be almost unrecognisable. On August 15 the Orontes was 

 suspected to be, and the Euphrates was certainly, double, 

 whilst, later, the Amenthes was seen to be broad and 

 diffuse. M. Antoniadi suggests that the pale greyness of 

 the darker regions may be due to the interposition of very 

 light clouds or of a mist in the Martian atmosphere. Both 

 MM. Ou(§nisset and Jarry-Desloges also direct attention to 

 the urTusual paleness of' the dark regions of the planet 

 during the past few weeks, and each account is illustrated 

 by reproductions from the original drawings showing 

 various aspects of the planet. 



