714 



NA TURE 



[November 25, 1922 



there is no evidence that a day of exceptional 

 visibility is more likely to be followed by rain than 

 a day of low visibility; the author says, rather the 

 reverse. Only one-third of the days with visibility 

 21 miles or more were followed by rain, while with 

 visibility less than 13 miles one-half of the days were 

 followed by rain. A discussion on visibility at a 

 meeting of the Royal Meteorological Society last 

 December has helped much to an understanding of 

 the subject. Exceptional visibility or " nearness " 

 is by no means a common feature, and for a test as 

 a sign of coming rain, it seems to require different 

 handling from that given by the author. 



Weather in Korea. — The annual reports of the 

 meteorological observatory of the Governor-General 

 of Chosen for the years 1918 and 1919 have recently 

 been received. They give hourly meteorological 

 observations at Jinsen (Chimulpo) mostly from 

 European self-recording instruments, and also daily 

 means and extremes. At fourteen branch-stations 

 the several meteorological means are given for each 

 4 hours for each month and for the year. For several 

 auxiliary stations the mean highest, mean lowest, 

 and mean temperatures are given, as well as the 

 amount of precipitation and days with rainfall, also 

 the maximum precipitation in a day at each station 

 for each month, and the dates of first and last frost 

 and first and last snowfall. There is great variation 

 of temperature and rainfall at the different stations 

 and at different seasons consequent on the varying 

 heights of the stations and the vastly different 

 exposures, the country being generally very moun- 

 tainous. The auxiliary stations supplying data for 

 the climatological investigation of the peninsula 

 numbered 203 at the end of 1919. With the object 

 of securing data for the investigation of thunder- 

 storms four hundred head masters of the ordinary 

 Korean schools report all the phenomena which 

 accompany the storms, and this will be continued for 

 three years from 1918, confining the reports to the 

 warm season from April to September. The con- 

 tinuity of these observations, year after year, adds 

 greatly to the general knowledge of the world's 

 meteorology. Magnetic observations, which formed 

 part of the ordinary routine, have for the time been 

 suspended owing to the destruction of the quarters 

 by a severe storm in September 1919. 



The Rocky Mountain Oil-field. — New informa- 

 tion concerning oil possibilities of the great Rocky 

 Mountain Field of North America is always of interest, 

 and particularly so in connexion with Montana, which, 

 compared with the adjacent State of Wyoming, has 

 up to the present yielded surprisingly poor results. 

 Stratigraphically and structurally there is much 

 territory in Montana which should prove favourable, 

 though large areas are at present unprospected for 

 oil and gas. Mr. W. T. Thorn, Jr., in a brief report 

 published recently, has thrown much light on at 

 least one interesting district, that of the Crow Indian 

 Reservation, in Big Horn and Yellowstone counties, 

 the southern part of the State. Some 3000 square 

 miles of this Reservation, lying to the north and east 

 of the Big Horn-Pryor Mountain uplift (forming the 

 dominant regional structure), offer the best prospects, 

 and within this area a local uplift known as the Soap 

 Creek Dome is being developed ; most of the oil 

 obtained has been won from the Amsden forma- 

 tion, a shale-and-sand series of Pennsylvania 11 age. 

 The underlying Madison Limestone — a particularly 

 pure limestone of Mississippian age and well developed 

 in central and southern Montana — has also yielded 

 oil at Soap Creek. Although no mention is made 

 of the quality of the oil obtained during develop- 



NO. 2769, VOL. I io] 



ments, it may be stated that great variation in gravity 

 is the general characteristic of the petroleum obtained 

 in southern Montana and northern Wyoming ; such 

 variation is dependent largely on the different 

 geological horizons from which the oil is drawn, and 

 in many instances, owing to the complexity of 

 structure and widespread faulting, the nature of the 

 oil changes with almost surprising rapidity from well 

 to well. The future of the whole Rocky Mountain 

 Field as regards oil production centres largelv in the 

 States of Wyoming, Colorado, and Montana, and 

 although up to the present Wyoming has produced 

 more than 90 per cent, of the oil obtained, the 

 prospects for the other two States are by no means 

 discouraging, as the present Crow Reservation bulletin 

 reveals. 



Grain Size in Photographic Emulsions. — The 

 nature of the developable image, the cause of sensitive- 

 ness, the relation between the size of the particles 

 in a photographic emulsion and its sensitiveness, 

 and other allied problems, have received a great 

 deal of attention at the hands of several investigators 

 during the last few years. This work has led to 

 various hypotheses, which have been noted from time 

 to time in our columns, all of which have not been 

 generally accepted. Messrs. E. P. Wightman, A. P. H. 

 Trivelli, and S. E. Sheppard, of the Research Labora- 

 tory of the Eastman Kodak Company, publish the 

 first of a series of papers entitled " Studies in Photo- 

 graphic Sensitivity " in the October number of the 

 Journal of the Franklin Institute, in which they 

 propose to examine these hypotheses, to note wherein 

 they lead to similar conclusions, and so far as possible 

 to test experimentally between them. The present 

 paper is on the distribution of sensitivity and size 

 of grain in photographic emulsions. The authors 

 discuss the existence and nature of statistical variation 

 of sensitivity among silver halide grains, and the 

 relation of this variation to the density-exposure 

 function. It is concluded that under certain condi- 

 tions the first derivative of the density-exposure 

 function will correspond with the intensity-variation 

 function or curve. The results of experimental 

 determinations of grain-size-frequency curves are 

 noted, and correlated with sensitometric data. The 

 decisive influence of the grain-size distribution and 

 limits on the " speed " and other sensitometric 

 variables is discussed in relation to the " quantum " 

 and the " photocatalytic " theories of grain sensitive- 



" Electrets," the Analogues of Magnets.-- 

 For the last three years Prof. Honda and his pupils 

 at the University of Sendai have been investigating 

 the conditions under which rods of solid dielectrics 

 permanently charged with positive electricity at one 

 end and with negative at the other could be produced. 

 In the most recent work of Mr. M. Sato, described 

 in the June issue of the Science Reports of the Uni- 

 versity, a tube containing a molten dielectric has 

 electrodes at its ends connected respectively to the 

 positive and negative terminals of an electrical 

 machine. The dielectric is allowed to solidify slowly 

 from each end, the middle portion remaining liquid 

 longest, and when the rod of solid dielectric is 

 extracted from the tube, it is found to be charged 

 positively at one end and negatively at the other, 

 and the charges will continue apparently for years. 

 If the rod is cut into short lengths each length is 

 permanently charged, the amount of the charge 

 being proportional to the distance of the length 

 from the middle of the rod. According to Mr. Sato, 

 these " electrets " are due to the ions held in fixed 

 positions by the solidification of the dielectric. 



