348 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 31 



severe, say, as that which on September 5, 1732, damaged some 300 

 houses in Montreal alone, resulted in the death of at least one, and 

 the injury of many under circumstances overwhelmingly less likely 

 to prove serious than those which obtain in the same locality to-day. 

 Those engaged in construction work in this country will some day 

 more generally consider the known precautions with regard to 

 earthquake tremors which should be observed in varying degree de- 

 pending on the nature of the ground concerned. 



Our subject is not earthquakes, however, but the earth. Astron- 

 omy first taught us something of our earth as a whole. That science 

 was partly responsible for and greatly assisted the voyage of Colum- 

 bus, which demonstrated that one could cross the Atlantic without 

 at least falling off. The idea that the earth is spherical is now gen- 

 eral except in Zion City, 111. Astronomy measures for us the radius 

 of our planet, finding it to be about 4,000 miles, the circumference 

 being nearly 25,000 miles. Astronomy mothered geodesy, which has 

 demonstrated that the earth is a spheroid of the oblate order. The 

 remark about the orange which invariably follows in the schoolbooks 

 leaves one with an uneasy sense of a religious controversy in this 

 connection. 



The shape and size of the earth being known, mathematical physics, 

 also an offspring of astronomy, informs us that the average density of 

 the earth is 5.6 (in c. g. s. units). It weighs, therefore, volume for 

 volume, 5j% times as much as water. This statement may be taken 

 as a convenient crossover to geology. Physics and allied sciences are 

 able to tell us only average values for conditions within the earth — 

 the average density, the elasticity as a whole, the mean value of 

 gravity ; the details are notably lacking. The geologist is concerned 

 with surficial details. Informed that the earth, as a whole, weighs 

 a little more than S^/o times as much as an equal volume of water, 

 he states that the granite, marble, limestone, dolomite, etc., which 

 form the bulk of our surface rocks weigh only about half as much, 

 volume for volume, as the earth as a whole and that, therefore, there 

 must be much denser material below. The questions throng: How 

 far down do the surface rocks extend? "VVliat distribution of den- 

 sities exists within the earth? In what physical state do the mate- 

 rials exist, solid, liquid, or gas? We are curious to investigate what 

 lies within the earth beneath. 



The miner has the terse expression, " Beyond the pick it is dark." * 

 If this be so, how far may we penetrate below the surface. The 

 deepest mine in the world ^ is the St. John del Rey, in Brazil. This 



*Ambronn. Richard, Elemeuts of geophysics (translation by Margaret C. Cobb), 372 pp. 

 (see p. 1), McGraw Hill, New York, 1928. 



" Science News, The age and depth of mines. Science, No. 1541, vol. 60, p. vlil, July 11, 

 1024. A short article preparea by the U. S. Department of the Interior. 



