A.— MATHEMATICAL AND PHYSICAL SCIENCES 31 



constituting the various beds. Salt domes under alluvial deposits, for 

 example, are in this respect suitable structures, and the location of many 

 such domes was the first achievement of the seismic method. It has also 

 been employed with valuable results in determining the underground 

 contours of limestone anticlines and deep-seated granitic basements at 

 depths of several thousand feet. We may notice that the method is rarely,, 

 if ever, able to reveal directly the presence of the mineral actually sought. 

 It does not find oil as such, for example, but it may discover those rock 

 structures with which there is a high probability of oil being associated. 

 For the essential knowledge of such associations applied geophysics 

 depends on geology. 



Concerning the portable seismographs and time recorders which are 

 the tools of the method, I must here be content with remarking that many 

 ingenious instruments have been made available latterly in this country 

 as well as abroad. Most of them record the displacements of the earth's 

 surface at the place where the instrument is located ; a few record the 

 velocity, as in the Galitzin seismograph. There are claims too, rather 

 shrouded in secrecy, of the successful use of accelerometers. I mention 

 this because it seems to me that this is the direction we ought to pursue 

 in future developments. The diffracted disturbance, calculated by 

 Jeffreys, begins with zero displacement and zero velocity, but its initial 

 acceleration is finite. Consequently such disturbances may be expected 

 to display much sharper and precisely determinable onsets in the record 

 if this is of acceleration instead of either displacement or velocity. Greater 

 accuracy of timing would thus be secured, leading to more precision in 

 the results ; probably, also, smaller and less expensive explosive charges 

 would be required. The application of piezo-electricity ^ obviously 

 suggests itself. 



The Magnetic Method. 

 We pass now to the magnetic method. In actual practice it is the 

 simplest and least costly. It consists of measuring, with suitable portable 

 magnetometers, local variations of components of the earth's magnetic 

 field, usually the vertical and horizontal intensities. The instruments 

 which have been designed for the purpose enable observations to be made 

 quickly, so that a large number of stations can be occupied, and a wide 

 area covered, in the course of a single day. Under suitable conditions, 

 therefore, much information regarding underground structure may be 

 obtained by means of a survey lasting only a relatively short time and 

 involving comparatively little expense. But it should be pointed out that 

 this apparent economy has sometimes led to the method being employed 

 on problems for which it is at present unsuitable, and to claims being made 

 as to its performance which are doubtful. It is necessary to bear in mind 

 that the basis of magnetic surveying is the differentiation of rocks in 

 respect of magnetic susceptibility, and the consequent discontinuities of 



^Prince Galitzin in 1915 described an apparatus depending on piezo-elec- 

 tricity for the direct measurement of accelerations, and subsequent investigators 

 have used the method also ; but seismographs of this type do not seem to have 

 come into common use, at any rate as portable instruments. 



