70 



JVA TURE 



[November iS, 1909 



Prussian Geodetic Institute and of the Central Bureau 

 of the International Geodetic Association. The same 

 chief directs them both, and a visitor sees no obvious 

 line of demarcation between the two. There is, how'- 

 ever, no need to inquire minutely how much is 

 Prussian and how much is international; the resources 

 and e.\perience of both arc freely available when 

 advice and cooperation are desired. 



The last series of pendulum operations in India 

 had been brought to a close in 1871 with the tragic 

 death of Captain Basevi, who lost his life from ex- 

 jjosure while working with the Royal Society pendu- 

 lums at great altitudes in the Himalayas. When the 

 (jovernment of India resolved to undertake a new 

 and more extensive series, they began by asking the 

 advice of Prof. Helmert, chief of the Central Bureau. 

 He recommended the use of half-second pendulums 

 (if the von Sterneck pattern, offered to obtain them 

 from the makers in Vienna who had supplied the 

 Potsdam instruments, and to standardise them at 

 Potsdam. In 1902 Major Burrard and Captain 

 Lenox-Conyngham went to Potsdam to study the use 

 of the pendulums, and Prof. Haasemann, to whom 

 the standardisation had been entrusted, gave up the 

 whole of his time to their instruction. The pendu- 

 lums were then swung at Kew and at Greenwich, 

 were taken to India, and set to work; and, lastly, it 

 was arranged that Dr. Hecker, of Potsdam, return- 

 ing from his second " gravity-voyage," should join 

 the Indian pendulum partv wherever they might 

 happen to be, and swing his Potsdam pendulums in 

 India, to provide a final control of the circuit Potsdam 

 — Kew — Dehra Dun — Potsdam. 



The admirable results of this cooperation are visible 

 on every page of Major Lenox-Conyngham 's memoir. 

 He started with an equipment alreadv well tried. He 

 could face the peculiar difficulties of Indian pendulum 

 work with the confidence that he knew all about the 

 ordinary troubles; and, by no means least, he can 

 describe his work in a way which tuakes it perfectly 

 easy to read in conjunction with German work, for 

 they are written in the same language, though in a 

 different tongue. 



It is well known that an exceptional difficulty has 

 embarrassed the survey of India for many years. 

 How much does the enormous mass of the Himalayas 

 and the Tibet plateau to the north, how much does 

 the enormous deficiency of mass in the deep ocean 

 to the south, affect the direction of gravity in India? 

 India was the birthplace of Pratt's celebrated hypo- 

 thesis of "compensation." The general principle, 

 that an excess of matter above the sea-level is com- 

 pensated by a deficiency of density below, has been 

 established in the Caucasus and the Tyrol. The fact 

 that gravity is normal, not deficient, over the deep 

 oceans was strongly suggested by Faj-e when he 

 showed that on solitary islands gravity is in excess 

 by an amount corresponding to the mass of the 

 island above the ocean floor ; it has recentiv been 

 established by Dr. Hecker 's elegant application of 

 Mohn's method — the comparison of the mercurial 

 barometer and the boiling-point thermometer at 

 sea. 



Only in India has the establishment of the principle 

 of compensation suffered a series of small reverses. 

 Colonel Burrard's series of latitude determinations, 

 published a few years ago, showed that a hidden 

 chain of excessive density runs parallel to the Hima- 

 layas to the south, which very much complicates the 

 situation. Major Lenox-Conyngham 's pendulums 

 confirm the existence of this chain, and brine to light 

 a new feature — that between it and the Himalayas 

 there is a "ditch " alone: which gravity is in defect; 

 and, lastly, along the fringe of the mountains their 

 NO. 2090, VOL. 82] 



attraction is by no means completely compensated. 

 Five stations in the Himalayas and two in the 

 Baluchistan hills were occupied. " .\t all these points 

 a deficiency of density is revealed, but in no case 

 does it amount to total compensation. . . . Under all 

 the submontane and mountain stations there appears 

 to be a deficiency which is nearly constant in 

 amount, and is not proportional to the height of the 

 station." 



But these interesting results touch only the fringe 

 of the Himalayas, and they are not to be taken as 

 indicating a general failure of complete compensation 

 for the mountain mass as a whole. Before this can 

 be either atiirmed or denied it will be necessary to 

 get observations further north ; and it is interesting' 

 to note that, as Major Leno.x-Conyngham points out, 

 it is more important to get well in among the moun- 

 tains than to climb to excessive altitudes, where the 

 conditions make accurate work almost impossible. 

 Unfortunately, access to Tibet is forbidden to British 

 subjects, including its own scientific servants, by the 

 Indian Government; nor is it easy to disguise a 

 pendulum equipment as a praying-wheel. 



The technical details of modern pendulum work 

 are highly interesting, in particular the correction 

 for the flexure of the pendulum support. The present 

 practice is a development of Captain Rater's method 

 of the inverted pendulum initially at rest, which is set 

 swinging by the motion of the stand. In its actual 

 shape it is the invention of Prof. Schumann, of the 

 Prussian Geodetic Institute. \n auxiliary pendulum 

 is mounted with its knife-edges parallel to those of 

 one of the set; it has an adjustable weight on the 

 bob by which the times of vibration can be made 

 equal. If one is set swinging, the pull of its knife- 

 edges on the agate plates will rock the stand and set 

 the other swinging; over a considerable range the 

 ratio of the amplitudes of the driven and the driving 

 pendulum increases proportionally to the time, and a 

 simple relation connects this ratio with the virtual 

 increase in the length of the pendulum due to the 

 flexure. It is thus a simple matter to determine the 

 correction for flexure of the stand and pillar at each 

 station, and a prettv illustration of the need of the 

 correction is given by those series which were begun 

 upon a concrete pillar newly cast, and show un- 

 mistakably the gradual stiffening of the pillar as the 

 concrete hardened. 



The pendulum observations are differential, and it 

 is necessary to choose a base. Kew Observatory was 

 selected, because it had been the base for the earlier 

 Indian series, and the National Physical Laborato'-y 

 gave valuable assistance in the observation of the 

 base series and a re-determination of the constants 

 of the pendulums for verification. .At the suggestion 

 of the Astronomer Royal, Greenwich was chosen as 

 a secondary base. K remarkable feature of the report 

 is the exhaustive appendix by Mr. \. Strahan on the 

 geological strata underlying Kew and Greenwich, 

 and their height above the " Palaeozooic floor." With 

 these refinements of observation and discussion the 

 observed differences in the value of g at Kew and 

 Greenwich become fairly accordant with the theo- 

 retical differences, and warrant the conclusion that 

 " henceforward the pendulum may prove as satisfac- 

 tory in practice as it has always been attractive in 

 theory." 



Dr. Drygalski's Antarctic expedition had the bad 

 fortune to be frozen in just outside the Antarctic 

 circle, and he was unable to reach Antarctic land. 

 His pendulum operations were therefore confined to 

 the island of Saint Vincent in the Cape Verdes, to 

 Kerguelen Island, and to a station on the ice near 

 the winter quarters of the Gauss. ."M the two former 



