July i8, 191 8] 



NATURE 



39: 



t^ives brief particulars of a number of installations, 

 which are already woiking, and of others which it is 

 hoped to erect when capital and labour are available 

 for the purpose. 



The amplitude of the oscillations produced in a 

 singing arc depends on the curvature of the current- 

 voltage characteristic curve. If i^ is the constant cur- 

 rent upon which the variable current of effective value 

 r is superposed, it is shown {Revue generate de 

 I'Electriciti, April 20) that ii = kpijr, in which fe is a 

 coefficient, p the curvature of the characteristic curve 

 at the region over which the oscillations are taking 

 place, and r the resistance of the oscillatory circuit. 

 In this equation i^ depends on i,, and also on p and r, 

 but if i, is not very large, i, may be considered as 

 nearly equal to the current that would be obtained 

 with i,=o, and the equation wx>uld give to a first 

 approximation the effective value of the oscillating 

 rurrent in the singing arc. 



In a lecture at the Technical College, Munich, 



Dr. A. Traube read a paper on the production of 



coloured photographs by a process which he calls 



Uvachromie." Three photographs are taken through 



-Liitable colour-filters, the negatives being copied on 



■ rdinary kinematograph films. After fixing, these 

 tilms are treated in a bath so as to form a chemical 

 lompound that readily absorbs colour material. The 

 three component pictures can be coloured and dried in 

 about twenty minutes, and they are then placed in 

 register upon one another, and the coloured photo- 

 graph is complete. The new process has the advan- 

 tage that as many copies may be made as are required, 

 the process being rapid and simple. The photograph 

 retains its sharpness of definition, and corrections in 

 colour can easily be made by replacing one or other 



if the component films in the colour solution for the 



ppropriate kngth of time. 



The May (1918) issue of Navigazione interna (Pro- 

 paganda deir Associazione Nazionale del Congress! di 

 Navigazione) gives particulars of the work of the 

 Hydrographic Office of the Po during the period 

 X914-17. In spite of the w-ar, much useful work has 

 been done by the institution with regard to rainfall 

 observation (the number of observation stations both in 

 tlie mountains and plains has been increased and new 

 methods of observation have been initiated), hvdro- 

 ^laphy, levelling surveys, measurement of flow of 

 -1 reams, forecasting of floods and dry periods, the 

 measurement of the turbidity of streams, temperature 

 measurements on the waters of the Po, investigation 

 (if subterranean streams, etc. I'iainfall observation 



i.id investigation, in particular, have received a great 



ieal of attention. It is proposed to studv Swiss 

 methods of rainfall and snowfall observation, using 

 various modern types of recording instrument, and 

 to correlate and co-ordinate results. Glacier study has 



1 1 so had the attention of the authorities, and results 

 I considerable importance as to the formation of 

 laciers and their influence on rainfall and tempera- 



Lire in valley regions should be anticipated when this 

 Aork has been placed on its final basis. 



Attention may be directed to a Bulletin just issued 

 by the Engineering Experiment Station of the Uni- 

 versity of Illinois, entitled "Percentage of Extrac- 

 tion of Bituminous Coal, with Special Reference to 

 Illinois Conditions." It is well known that in coal- 

 I mining it is never possible practically to extract the 



f whole of the coal from any given seam, and that a 

 certain percentage is always left behind and lost; fur- 

 thermore, such loss is admittedly greater on the aver- 

 age in the United States than it is in Great Britain, 

 one of the main reasons being the low price of coal 

 1 the United States; "the coal-mining engineer of 

 NO. 2542, VOL. lOl] 



America accordingly has not had as his problem the 

 development of methods of extraction which would 

 result in the largest percentage of ultimate recovery, 

 but rather the development of methods which would 

 result in the lowest cost of production." From a table 

 given in the present Bulletin it appears that the per- 

 centage of recovery of the entire seam ranges from as 

 much as 97 per cent, in the George's Creek Field of 

 Maryland to as low as 50 per cent, in Central Illinois. 

 The subject necessarily requires discussion in detail, 

 since the amount of coal left behind def>ends in each 

 case upon a large number of conditions, the nature of 

 the seam, its thickness, depth from surface, inclina- 

 tion, character of roof and floor all having to be 

 taken into account, whilst the amount of damage done 

 to the surface and the monetary value of such damage 

 have also to be considered. Although American con- 

 ditions of coal-mining are very different from those 

 prevailing in this country, the Bulletin will well repay 

 perusal by British coal^miners, especially if it arouses 

 sufficient interest to cause a similar investigation to 

 be undertaken in this country. 



According to a note in the Chemical Trade Journal 

 for June 29, a new radio-active element of consider- 

 able emissive power has been detected in the residue 

 frpm pitchblende, w-hich forms the raw material em- 

 ployed as a source of radium. This residue w^as sub- 

 jected to treatment which finally left undissolved only 

 the members of the tantalum group ; and this in- 

 soluble remainder showed a radiation, at first slight, 

 but gradually increasing largely, which proceeded 

 mainly from the evolution of actinium, and indicated 

 the presence of the new element " protactinium." 

 Experiments for the separation of the element are to 

 be undertaken. The period of semi-disintegration 

 probably fluctuates between 1200 and 18,000 years. 

 The information is based on statements published in 

 the Milnchner Neueste Nachrichten. 



OUR ASTRONOMICAL COLUMN. 

 Discovery of Wolf's Periodic Comet. — M. 

 Jonckheere, who has been searching for the return 

 of Wolf's periodic comet since May 4, discovered it 

 on July 9 at loh. 45m. G.M.T. with the 28-in. equa- 

 torial at Greenwich. The comet was at the time of 

 discovery between the 15th and i6th magnitude; and 

 about 9" in diameter. It is about +50S. in R.A. and 

 15' north of the place given by M. Kamensky's orbit 



I (A.J. 729). On Julv 10 the magnitude was estimated 

 as 15th, and on July 12 as 14th. The comet was dis- 

 covered in 1884, w^hen it was of 8th magnitude, and 

 was observed in the returns of 189 1, 1898, and 191 1, 

 but not in 1905. 



The New Star in Aquila. — The new star has 

 varied but little in brightness during the past week, 

 and , was of approximately the 4th magnitude on 

 July 14. The spectrum, how'ever, has shown a further 

 approach to the nebular stage. On July 13 Prof. 



I Fowler observed that in the visible spectrum the en- 

 hanced lines of iron were represented only by very 

 feeble lines at 517 and 532, while the line about 501, 

 which would appear to be the chief nebular line, was 

 scarcely inferior in brightness to He- The line about 

 Hy also appeared to have gained in relative bright- 

 ness, as if the nebular line 4363 had made its appear- 

 ance. The band at 464 was bright and broad, and a 

 faint band on its less refrangible side was probably 

 4686. There was also a faint band about the position 

 of the helium line 4471. The group of three bright 

 lines in the region of D was reduced in intensity. 

 It would seem that the loss in magnitude due to the 

 fading out of the enhanced lines and the reduced 



