PRESIDENTIAL ADDRESS SECTION A. 33 



quake waves in various directions round and through 

 the earth. 



I should like to add that in my opinion the recent pro- 

 foundly important discoveries of the arrangement of the mole- 

 cules in crystals is likely to lead at an early date to our being 

 able to calculate the elastic constants and strength of solids even 

 at the enormous pressures and high temperatures that must 

 exist in the interior of the earth. If such be the case, the whole 

 of our knowledge of the interior of the earth will attain an 

 exactitude quite unexpected only a few years ago. 



This summary of methods will show how impossible it is 

 for anyone to attempt anything but the shortest outline of some 

 of the more certain results that have been yielded by them, or 

 to make more than passing reference to the need there is in 

 many cases simply to suspend judgment. 



The size and shape of the earth have been measured with 

 something like accuracy, only by geodetic surveys made on con- 

 tinental areas. Such measurements vary slightly, but the general 

 result is that the land surfaces, when reduced to a mean level, 

 are very approximately those of a flattened ball or spheroid 

 such as would be produced by spinning an ellipse round its 

 shorter axis, the equatorial radius or semi-axis being 6,378.2 km. 

 very nearly, or 3,963.3 miles, and the solar radius being 1/298 

 part shorter. It is worth pointing out that the radius of curva- 

 ture of the oceans has not been directly determined. It is known 

 from dynamical theory that if the earth consisted of a series of 

 concentric shells, each of uniform density, the free surface would 

 be a flattened spheroid such as has been described. But a direct 

 determination of the curvature of the ocean by a goedetic chain 

 of triangles extending over some of the Pacific Islands would, if 

 feasible, yield information of great interest. 



The method of such geodetic surveys is, of course, a well- 

 known one, but as it has an intimate bearing on some questions 

 dealing with the strength and movements of the earth's crust 

 that will come up for our consideration later on, I may be par- 

 doned if I refer to it briefly. A level base line several miles 

 long is carefully measured, and from its ends the angular posi- 

 tions of two marks set up on neighbouring hilltops are accurately 

 determined by a theodolite. The base line is thus the base of 

 two triangles that have the hilltop marks as vertices. The line 

 joining these serves as a new base from which one or two other 

 points are determined. So the process of triangulation goes on, 

 the triangles spreading over the country, and the individual 

 sides being ultimately usually about 30 miles long. The vertices 

 of the triangles are almost invariably beacons set up on promi- 

 nent mountain peaks. I need hardly say that the highly accur- 

 ate and extensive geodetic sun'ey of South Africa which we owe 

 to the energy and skill of the late Sir David Gill and of Colonel 

 Sir Richard Morris is one of the finest examples ever effected. 

 To find the shape of the earth we must also by star observations 



