Kay 15, 1914] 



SCIENCE 



705 



puted which would have far greater accu- 

 racy than any now existing. 



The theory of isostasy has been applied 

 in several other countries in the reduction 

 of deflections and of gravity, but not to 

 any great extent. In Switzerland the new 

 method makes the observed and computed 

 gravity agree more closely than do other 

 methods which do not consider isostasy. 

 In India the theory of isostasy has been 

 applied and the preliminary results indi- 

 cate that that country is not so completely 

 compensated as is the United States.^^ The 

 complete reductions have not been made. 

 Their results will be of world-wide interest 

 and importance. 



The reduction, by the new method, of all 

 the gravity stations of the world should be 

 made in order that additional data may be 

 available for studying the distribution of 

 the materials in the earth's crust in differ- 

 ent parts of the world, and because the 

 results would make it possible to compute 

 a better shape of the earth. There is no 

 necessity for delaying this reduction. Mr. 

 Tittmann, the superintendent of the Coast 

 and Geodetic Survey, in his report to the 

 conference, at Hamburg, in 1912, of the 

 International Geodetic Association, stated: 



It is hoped that the International Geodetic As- 

 sociation in the near future may reduce all the 

 available gravity stations of the world by this 

 method, for it is reasonably certain that a value 

 for the flattening of very great precision could be 

 deduced from the results. 



While each of the great nations and 

 many of the smaller ones are making gravi- 

 metric surveys, yet there are many sections 



13 In a letter from Col. Burrard, published in 

 Nature, May 8, 1913, he makes the statement: 

 "Both the pendulum and plumb-line observers find 

 the attraction of the Himalayas to be largely com- 

 pensated, whilst the Windhyan Mountains are not 

 compensated at all." It should be noted that the 

 Himalayan range is about 4 miles in height, and 

 the Windhyan range only 0.6 mile in height. 



of the globe which have very few or no 

 gravity stations. Some great institutions 

 may, in the not distant future, see the im- 

 portance of a world-wide gravimetric sur- 

 vey and start out parties to make one. 



The geodesist is in great need to-day of 

 an apparatus for measuring gravity accu- 

 rately on a vessel at sea. We have no accu- 

 rate values over the vast ocean areas. Dr. 

 Hecker's gravity determinations at sea, with 

 the boiling point thermometer apparatus, 

 are subject to uncertainties as large as the 

 largest new method anomalies of gravity 

 in the United States, that is, between 0.050 

 dyne and 0.100 dyne. Gravity at sea 

 should have actual errors not greater than 

 about 0.010 dyne. 



Some of the geological questions may or 

 may not be answered by a consideration of 

 isostasy,^* but with isostasy an established 

 fact, at least for large areas, the problems 

 should not be as difficult as when various 

 theories were held as to the degree of rigid- 

 ity of the earth's crust. 



One of the problems is as to the manner 

 in which the earth's surface assumed an 

 irregular shape. This is probably the most 

 difficult one to answer. Of the theories 

 proposed by a number of great men of sci- 

 ence, none seems to be entirely satisfying. 

 The theory of the unequal contraction of 

 the outer layers of the earth during the 

 change from the liquid to the solid state 

 sounds reasonable until we consider that 

 the thickness of the crust which could be 

 affected is only about 70 mil&s, according 

 to the theory of isostasy. There are exten- 

 sive areas on land which average several 

 miles in height. Also the great deeps in 

 the oceans for considerable areas have 

 average depths as great as four miles. 



14 See ' ' The Eelations of Isostasy to Greodesy, 

 Geophysics and Geology," by J. F. Hayford, 

 Science, N. S., Vol. XXXIII., No. 841, pages 

 199-208, February 10, 1911. 



