ISOSTASY, The word isostasy expresses an idea which has been developing gradually in definiteness for a century.
Pratt, an English geodesist, reached the con clusion early in the 19th century that the deflec tions of the vertical apparently produced by the Himalayas are much smaller than they would be if those mountains and the material beneath them were of the same average density as the material beneath the plains of India, Pratt's reasoning was based entirely upon the well established law of gravitation, namely, that the attraction between any two small portiorp of matter is proportional to the product of their masses and inversely proportional to the square of the distance between them. He computed roughly the attraction, due to the mass of the Himalayas, which would act upon a plumb bob hung at various points over the plains of India and computed the deflection of the plumb line which would be produced by this nearly hori zontal attraction tending to pull the bob toward the mountains. The deflections of the plumb line as observed at these points by geodetic methods were much smaller than the deflections which Pratt thus computed. He offered the possible explanation that the material under lying the mountains is of smaller average den sity than that underlying the plains, or, in other words, that the excess of mass visible in the mountains is compensated by a defect of den sity, and therefore of mass, in the material be low the mountains. This was the beginning of the idea now called isostasy.
Many years later various American geolo gists noted, as they believed, a prevailing tend ency for large portions of the earth's crust to rise as load was removed from the surface by erosion, or to subside as load was added by de position. They reasoned that such a sensitive ness to change of load indicates that there is a close approach to equality in the total load un der the various equal areas of the earth's sur face. Their idea may be roughly expressed by saying that the total weight of any conical mass of matter having the earth's centrefor apex, and any one square mile of the earth's surface as its base, is nearly the same as that of any other similar conical mass having some other square mile for its base.
Maj. C.E. Dutton, in an address before the Philosophical Society of Washington in 1889, coined the word isostasy, and then and there presented the idea which the word repre sents so clearly and marshalled the Avidenee in favor of the validity of the idea so skillfully a to attract general attention. From that day to the present the discussion of the idea and of its numerous important relations to geology and geodesy has been continuous and increasing in vigor.
From about 1898 to date (1918) the geodetic evidence that the condition called isostasy exists has been steadily gathered in a continuous in vestigation by the 'United States Coast and Geo detic Survey,— an investigation which is more vigorous in this line than that carried on in any other country. Two methods are used in that organization. The first is a refinement and extension of the method used by Pratt which has already been mentioned. The other method of observation consists in measuring with great accuracy the acceleration due to gravity at various points on the earth's surface swinging invariable pendulums. It is evi dent that if the material under the observing station is of very low density the gravitational force acting on the pendulum bob- will be smaller, and it will swing slower, than it would if the material were of normal density. Simi larly over material of high density the pendu lum will swing faster than over material of normal density. By properly refined methods of observation and computation the pendulum serves to indicate variations in density of the material underlying the region of observation. Geodesists in various countries have been espe cially active since about 1906 in developing the evidence in regard to isostasy.