NOTES 297 



that those who receive this rather expensive treatment at school do not, at 

 home, defeat its purpose by reading ordinary print or by attending the cinema ! 



The Reprint and Circular Series oj the National Research Council, U.S.A., 

 No. II, contains a survey of the outstanding research problems in geophysics, 

 prepared by the chairman of the several sections of the American Geophysical 

 Union. The chapter on Geodesy, by William Bowie, deals first with the 

 errors of geodetic surveys which start from different astronomic stations. 

 These are due to errors in the position of the plumb line caused by inequalities 

 of the earth's surface, and make it most desirable to base the survey of any 

 given area {e.g., of Europe) on a single datum. It also reviews the present 

 position of the Theory of Isostasy, which was first put forward by Archdeacon 

 John H. Pratt in the late 'fifties of last century. This theory supposes that 

 under mountain masses there are deficiencies of matter approximately 

 equal in quantity to the masses above sea level, while, under the bed of the 

 ocean, there is a corresponding excess, which approximately balances the 

 deficiency of matter in the mass of water above it. Investigations by John 

 F. Hayford, Chief of the Division of Geodesy of the U.S. Coast and Geodetic 

 Survey, published in 1909, showed that the depth affected by this excess 

 or deficiency of matter was roughly 122 kilometres. Further investigations, 

 published by the Coast and Geodetic Survey in 1917 and based on gravity 

 determinations, showed this depth of compensation {i.e., the depth at and 

 below which the materials of the earth are supposed to be in a state of 

 equilibrium) to be 96 kilometres. These observations also showed a decided 

 relation between the value of gravity and the underlying geologic formation. 

 This follows from the fact that the Cenozoic formation is approximately 

 10 per cent, lighter than the older geological formations, while Pre-Cambrian 

 rocks are decidedly heavier than normal. Thus pendulum observations 

 may be a means of indicating the nature of the strata close to the surface 

 of the ground — information which would have a considerable economic value, 

 e.g., in oil-boring operations. 



The review of the problems of Seismology by Harry F. Reid contains much 

 information of general interest. One of the outstanding problems of this 

 branch of science is the accurate determination of the time taken by 

 earthquake waves to pass from their point of origin to points on the earth's 

 surface at different distances from it. When this is known, it is possible to 

 trace out the paths and velocity of the waves through the earth and so to 

 obtain some knowledge of the physical condition of the earth's interior. 

 Records of earthquakes obtained at points more than about 110° from the 

 origin have, so far, proved very unsatisfactory, and it is still uncertain 

 whether this is due to the imperfections of the observations or, as Oldham 

 thinks, to the existence of a central core in the earth having different physical 

 properties from the material surrounding it. Professor Wiechert and his 

 collaborators claim that the transmission curves indicate the existence of 

 several shells in the earth where the velocity of propagation of the waves 

 suddenly changes. 



Among other chapters is a very important one by Louis A. Bauer on 

 Terrestrial Magnetism and Electricity. In this branch of geophysics one of 

 the great problems is the determination of the various systems of magnetic 

 and electrical forces which together make up the total terrestrial field as 

 observed at the earth's surface. It is known that the major portion of the 

 earth's magnetic field is due to an internal system of forces ; superimposed 

 on this is an external system probably due to electric currents circulating in 

 the earth's atmosphere and possibly also a system of vertical electric currents 

 which pass from the atmosphere into the earth and vice versa. The question 

 of the existence of these vertical currents is regarded by Sir Arthur Schuster 

 as one of the chief outstanding problems in terrestrial magnetism. The ocean 

 work of Prof. Bauer's department has been arranged specially for its solution 



