WHAT IS TERRA FIRMA? WILLIS. 399 



shown. The anomalies resulting from Bouguer's formula led Faye, 

 in 1880, to suggest that the correction for the attraction of the mass 

 between the station and sea level should be omitted. He reasoned 

 correctly that this mass is balanced and therefore is not equivalent 

 to a weight which is carried by the rigidity of the crust. He dis- 

 tinguished between the masses which are "compensated," or, as we 

 now say, balanced isostatically, and those which are in the nature of 

 loads superimposed upon the crust, and he wrote : 



It must be clearly understood that even if the thickness of the continents 

 above the sea has no place in the computation, this is not true, for example, 

 of the mass of the great pyramid of Egypt, if one were to observe the oscilla- 

 tions of the pendulum at its summit. In that case, after having reduced 

 the observation to the level of the sea, it would be necessary to subtract the 

 effect of attraction of the pyramid above the level of the ground. In the same 

 manner, if Bouguer had carried his pendulum to the summit of Pechincha, 1,500 

 meters above the level of Quito, it would be necessary to take account of the 

 attraction of this mountain upon the pendulum of Bouguer. 



It will be noted that Faye regarded the mountain Pechincha as a 

 mass upheld by rigidity, but considered compensation or balance to 

 be the condition of the larger mass below the plain of Quito. He 

 had thus been led by studies in geodesy to views which Gilbert 

 reached in 1889 by independent geological investigations. But Gil- 

 bert went further than Faye. He estimated the magnitude of the 

 mass which the earth would support rigidly and placed it tentatively 

 between 400 and 600 cubic miles. 



Faye's method was first employed by Putnam in 1895 and inde- 

 pendently by Gilbert, who collaborated with Putnam in the study of 

 gravity observations made in the United States under the Coast and 

 Geodetic Survey. 



Putnam's results were also calculated according to Bouguer's 

 method by Gilbert as well as by himself. The comparison with 

 Faye's method was greatly in favor of the latter, -as the values ob- 

 tained by Faye's method were much more accordant, when reduced 

 to sea level and the same latitude, than those obtained by Bouguer's 

 from the same observations. The comparison was so much to the dis- 

 advantage of the older method that it may be said to be no longer 

 worthy of consideration, and the very many results reached by it are 

 of relatively slight value. 



The accordance of results by Faye's method was so satisfactory 

 that Putnam and Gilbert may be credited with having established 

 beyond question the principle of isostasy as applied to the larger 

 features of relief of the earth's surface. The small number (35) of 

 stations considered by them and the limitations necessarily placed 

 upon the computations for corrections nevertheless lessen the value 

 of their estimates as to the load that the earth could bear rigidly. 



