March 27, 1913] 



NATURE 



95 



as insufficient to affect Prof. J. Joly's use of the 

 sodium in rivers and in the ocean as a measure of 

 geological time. 



From an educational point of view, no work issued 

 by the United States Geological Survey has been more 

 important than Professional Paper 71, by Mr. Bailey 

 Willis, entitled "An Index to the Stratigraphy of 

 North America" (1912). This memoir of 894 pages 

 Is accompanied by a coloured geological map of North 

 America on the scnle of 1 : 5,000,000, which includes, 

 not onlv Mexico, the United States, and Canada, but 

 Central America, the West Indies, Greenland, and 

 Iceland as accessories. We should recommend the 

 mounting of this map on large folding sheets, after 

 the excellent manner of the Oxford wall maps, so that 

 the whole or any of the four sections can be hung 

 up as required. The paper is much more than an 

 "index," since the formations are described in detail, 

 and the views of various authors as to their modes of 

 deposition are freely quoted. Canada is thus repre- 

 sented by her own authorities. The most striking 

 features on the map are the immense areas covered 

 by Tertiary volcanic rocks in the western Cordillera, 

 and, in contrast, the severe pre-Cambrian region of 

 the north-east, beyond the folded Mesozoic strata of 

 the Rocky Mountains. 



In common with all other great earthquakes, the 

 Messina earthquake of December 28, 190S, was fol- 

 lowed bv a large number of after-shocks. A record 

 of the shocks felt at Messina was kept by Mr. G. 

 Spadaro, then a student in the Nautical Institute in 

 that city. During the last four days of 1908, eighty- 

 seven shocks were felt, and during the following year 

 862. The majority of these shocks were, as usual, 

 very slight, but four (in December, 1908, and January, 

 1909I, were ruinous, and one (in July, 1909) almost 

 disastrous. Dr. Agamennone points out (in the 

 Rivista di Astronomia, &c, for last November) that the 

 distribution of these shocks in time does not follow 

 the simple law which, according to Prof. Omori, 

 governs the decline in frequency of Japanese after- 

 shocks, namely y = k/(x + h), where y is the number 

 of shocks in a given interval at time x. and h and k 

 are constants., for the monthly number is greatest in 

 March, 1909, and shows an increase in frequency 

 towards the end of that year. 



In the Revue g&nerale des Sciences for February 28 

 Prof. H. Devaux, of the University of Bordeaux, gives 

 a resume of his researches on the properties of thin 

 layers of oil spread on the surfaces of water and mer- 

 cury, a subject on which he has published a number 

 of papers during the last ten years. It appears that 

 the least thickness of oil which produces an appre- 

 ciable effect on the surface tension of either water or 

 mercury is much less than has been supposed. If a 

 film of oil the thickness of which is known from its 

 volume and area, is gradually thinned by increase of 

 its area, the surface tension of the surface it covers 

 has the value appropriate to oil until the thickness of 

 the film, i-iox 10- 7 centimetre, is less than the mean 

 diameter of an oil molecule, 1-13 x 10- 7 centimetre, as 

 determined by M. Perrin's method. Below this thick- 

 NO. 2265, VOL. 91] 



ness the film of oil does not affect the surface tension 

 of either water or mercury. 



The February number of the Journal of the Institu- 

 tion of Electrical Engineers contains the lecture on 

 permanent magnets which Prof. Silvanus P. Thomp- 

 son delivered at the meeting of the institution at 

 Glasgow last year. It occupies more than sixty pages, 

 gives a complete account of present-day knowledge on 

 the subject, and points ouj directions in which further 

 research is necessary. The author shows that the 

 most powerful and permanent magnets are made of 

 steels with about 6 per cent, of tungsten and 0-5 per 

 cent, of carbon, and have the ratio of length to breadth 

 large. After forging at as low a tempera- 

 ture as possible the magnets should be heated 

 to 900 C, cooled to 750° C, kept at that 

 for a time, and then cooled off. Hardening 

 is a repetition of this process down to 700 C, at 

 which temperature the magnets are to be plunged into 

 brine at 20 C. Maturing is done by boiling the mag- 

 nets for ten or twelve hours. Magnetisation is 

 effected by an electromagnet, and there is some advan- 

 tage in a few reversals. For extreme constancy the 

 magnetisation may be reduced by 5 or 10 per cent, by 

 subjecting the magnets to demagnetising forces. The 

 paper includes a bibliography which will prove ot 

 great use to future workers in this field. 



Mr. H. G. Seager, of Colwyn Bay, has devised an 

 "automatic control" for aeroplanes, which appears 

 well suited for the purpose of extricating an aviator 

 from the difficulties in which he is placed by a sudden 

 change of the conditions of either longitudinal or 

 lateral equilibrium, such as that due to a gust of wind 

 or a stoppage of the engines. It is perhaps not suffi- 

 ciently realised that the initial effect of the latter 

 cause is exactly represented by impressing on the 

 machine a wrench equal and opposite to that of the 

 propeller in steady motion, and if the propeller is 

 much below the centre of gravity the result will be 

 to turn the whole machine round until the air strikes 

 on the top of the planes and sends the aeroplane to- 

 earth. Mr. Seager employs a pendulum, so arranged 

 that anv finite displacement exceeding a certain limit 

 operates one or more air valves controlling pneumatic 

 motors, and these displace the controls through a finite 

 distance proportional to the number of valves operated 

 on, this number depending again on the displacement 

 of the pendulum. The arrangement has the obvious 

 advantage that the pendulum oscillations can be 

 damped out by friction, so that the apparatus can be 

 adapted to an inherently stable aeroplane without 

 interfering with its motion or control except in the 

 case of large disturbances. 



In " Untersuchungen iiber die Gezeiten der feste 

 Erde und die hypothetische Magmaschicht," a recent 

 publication of the Geodetic Institute at Potsdam, Dr. 

 W. Schweydar has made a very important contribu- 

 1 tion to the investigation into the nature of the earth's 

 interior. His discussion of a long series of horizontal 

 pendulum experiments throws considerable light on 

 the outstanding difference between the coefficient of 

 rigidity for th° earth indicated by the Chandler motion 



