ACADEMY OP SC.ENCES] rpjjj, J^g-p EIQ J IT YEARS 281 



upon it to such an extent after going to bed that it became necessary to devise various schemes 

 to divert his thoughts so that he might go to sleep. The economic problems of the war also 

 interested him greatly, and he foresaw that they would continue to trouble us after the war 

 ceased ; only a few days before his death he told his elder son that even though the war might 

 end soon, the world would be left so far out of adjustment that it would be necessary to economize 

 expenditures for years to come. 



During this period of old age, as well as in his more active maturity, Gilbert never under- 

 took any work in mining geology as a paid expert, never entered upon any organized movement 

 for public welfare, never held any public office apart from geological surveys, and with the 

 exception of brief periods of service as director on the boards of two Washington enterprises 

 of moderate dimensions, never took any official share in business matters. It is not to be 

 doubted that his clear discernment and his calm wisdom would have made him a valued asso- 

 ciate in many such relations, but they did not attract him. He followed political campaigns 

 with interest, but took no active part in them. He kept his own accounts accurately and was 

 a cautious investor and a careful spender; but business as such had no allurement for him. 

 He was unreservedly devoted to scientific work, which he always conducted in an absolutely 

 impartial manner, and in which he showed an extraordinary capacity for maintaining a sus- 

 pended judgment in problems that he did not regard as closed. If there be any such divisions 

 of science as pure and applied, his work belonged almost exclusively to the former. 



His face in these later years usually wore a thoughtful and serious yet placid expression, 

 but was often lighted with a smile. His hair was thin and white; his walk was still rapid, but 

 a slight stoop lessened his full height of 6 feet 1^ inches. His manner of speaking was like 

 that of his writing, clear, direct, concise. He had as always an even judgment, and in spite of 

 his scientific eminence he was never in the least arrogant ; on the contrary, he showed the gentle 

 serenity and the unaffected simplicity that accompany the wisdom born of a long experience 

 with nature and an untiring search for truth. 



LAST WORDS ON ISOSTASY 



Gilbert's latest study of isostasy was a short essay published at the end of 1913. 1 This 

 was almost his last work on any subject, for although it was followed in date of publication by 

 his report on the transportation and distribution of hydraulic-mining debris in California, 

 nearly all the work on those subjects had been completed previously. Since his three essays of 

 1895, great progress had been made in the isostatic problem by the geodosists of the Coast 

 Survey as well as by others in Europe. Stations for the observation of gravity had been much 

 increased in number, and the reduction of the observations had been greatly refined over earlier 

 methods by taking fuller account of the topographic irregularity of continental surfaces and of 

 inferred variations of underground density from place to place. As a result, the existence of 

 isostatic equilibrium in the earth 's crust had been shown to be more nearly perfect than was sup- 

 posed before; but let it be noted, in order that the reader shall not imagine the equilibrium to be 

 more precise than it is, that the average anomaly of gravity in the United States without regard 

 to whether it is in excess or in defect, had been placed at 630 rock-feet, and that several local 

 anomalies of 1,200 rock-feet were found; and those are no small quantities in view of the fact 

 that the mean altitude of the United States is only about 2,500 feet. Furthermore, a general 

 agreement had been reached with regard to the depth — the so-called depth of isostatic compen- 

 sation — at which the pressure should be uniform because of the inverse relation of rock density 

 and crustal thickness. The existence of such a level surface of uniform pressure had been 

 recognized in Gilbert's earlier work, but he had made no attempt to estimate its distance 

 below sea level. When it was shown to lie probably at a depth of 122 kilometers, and when 

 on the adoption of that depth the calculated gravity anomalies for 124 gravity stations and 

 for 765 deflection-of-the-vertical stations in the United States were made available for plotting 

 on a map, Gilbert carried the discussion of the meaning of the anomalies in some respects even 

 further than it had been carried by the geodesists, and in doing so showed an undi mi nished 



i Interpretation of anomalies of gravity. U. S. Oeol. Survey, Prof. Paper 85, pt. C, 1913, 29-37. 



