290 CJ. R. PUTNAM CONDITION OF THE EARTH 's CRUST 



otherwise than by a deficiency of matter below; secondly, that it increases 

 as the ocean is approached, and is greater on islands than can be ac- 

 counted for otherwise than by an excess of matter below." 4 



In 1881) Dutton proposed the word isostasy for this condition of 

 equilibrium in the earth's crust: 



"If the earth were composed of homogeneous matter its normal figure of 

 equilibrium without strain would be a true spheroid of revolution ; hut if 

 heterogeneous, if some parts were denser or lighter than others, its normal 

 figure would be no longer spheroidal. Where the lighter matter was accumu- 

 lated there would be a tendency to bulge, and where the denser matter existed 

 there would be a tendency to flatten or depress the surface. For this condition 

 of equilibrium of figure, to which gravitation tends to reduce a planetary body, 

 irrespective of whether it be homogeneous or not, I propose the name isostasy." 



The Transcontinental Gravity Measurements oe 1894 



In 1894 the writer made a series of measurements of the force of grav- 

 ity at stations which were favorably located for developing information 

 as to the problems of the earth's crust; it was the first extensive series in 

 the United States planned for this purpose, and included a station, Pikes 

 Peak, on one of the Eocky Mountain summits, and others on the western 

 plateaus and in the valleys, the geyser regions, and extending from the 

 Atlantic coast across the Appalachian Mountains and the central plains. 

 The work was initiated by T. C. Mendenhall, then Superintendent of the 

 Coast and Geodetic Survey, and in it the first extensive use was made of 

 the portable pendulum apparatus developed under his direction. The 

 expedition has an added interest to geologists from the fact that Gilbert 

 took part in it by making geological examinations about many of the 

 stations and by discussing the results. 



I will recur to the results of this work 5 and the methods of reduction 

 used. 



Hay ford's Eeduction of Gravity Observations 



In 1909 Hayford developed a method of reducing observations of the 

 force of gravity taking account, for the first time, of the attraction of all 



4 Walker : Nature, vol. xxxii, p. 486. 



" J The methods developed at this time permitted a considerable increase in rapidity of 

 field-work. The accuracy of the apparatus and methods and the constancy of the pen- 

 dulums are indicated by the fact that the mean periods of the three pendulums, A*, A\ 

 and A 6 , determined by the writer at the base station in Washington on 13 occasions 

 from 1894 to 1900, had an extreme range of but eleven ten-millionths of a second. Dur- 

 ing this interval the relative force of gravity was determined by the writer at 50 sta- 

 tions, including points in Greenland and Alaska and at the base stations in Europe, and 

 the same set of pendulums was transported approximately 50,000 miles. 



