A.— MATHEMATICAL AND PHYSICAL SCIENCES 25 



designed and adequate for the purpose — can be described as a post-war 

 development. It is of this that I wish to speak ; and, in this work which 

 has suffered from inadequate pubHcation, and the very nature of which 

 imphes collaboration, I shall not attempt to assign credit in any precise 

 manner. Except in one case. I single out Eotvos as the somewhat 

 reluctant pioneer of geophysical prospecting as it is known to-day. His 

 fundamental work on gravitation has made practicable what is still the 

 best-established and most precise geophysical method available. 



The Gravitational Method. 



1 do not think that Eotvos has yet received in this country the full 

 recognition which his work deserves. Possibly this is because the early 

 accounts appeared in rather inaccessible journals ; or, possibly, there were 

 real doubts concerning the validity of his claims. I remember, as a 

 student, hearing vaguely about his experiments — -and his name, without 

 anyone knowing how to pronounce it. In the same lectures we learnt 

 much fuller details of Boys' classic measurement of the constant of 

 gravitation, without realising how remarkably similar in essential form 

 the Eotvos and Boys instruments were. But the fact is that when Boys 

 was inventing and making the quartz fibres for his torsion balance to 

 weigh the earth, Eotvos had already tackled successfully the difficult 

 task of making robust and portable for field work another torsion balance 

 of not greatly inferior sensitivity. And while Boys was busy with his 

 measurements in a constant-temperature cellar, Eotvos was completing 

 the protection of his portable instrument against the temperature varia- 

 tions inevitable in the rigours of the field. A few years later he made 

 notably successful gravitational surveys on the frozen surface of Lake 

 Balaton, and on the Great Hungarian Plain ; but it was not until Shaw 

 and Lancaster-Jones ^ had demonstrated in 1923 that an Eotvos balance, 

 acquired for the Science Museum before the war, behaved according to 

 specification, that the remarkable nature of Eotvos' achievement began to 

 be appreciated here in this country. Truly Eotvos could claim in 1896,^ 

 following eight years' work, and only a year after the publication of Boys' 

 paper, that the extreme sensitivity of the methods he had invented for 

 measuring the space variation of gravitational fields had enabled him to 

 attack problems hitherto deemed to be unassailable. 



Even now I do not think it is well enough understood how small were 

 the effects which Eotvos measured under the unfavourable conditions of 

 field work. At the present day very fine measurement is common 

 enough, usually by methods of electrical amplification which have become 

 available ; but it may be well to place on record again what Eotvos did 

 forty years ago without such aids. We can illustrate this in a very striking 

 way. The earth's gravitational field, even apart from local irregularities, 

 is not uniform, or, rather, spherically symmetrical. Owing mainly to 



' Proc. Phys. Soc, vol. 35, pp. 151, 204. 



2 Annalen der Physik, vol. 59, p. 354. ' Die ausserst empfindlichen Methoden, 

 die ich besonders zur Messung der raumlichen Variationen dieser Krafte ersonnen 

 habe, machten es mSglich, mich solchen Aufgaben zuzuwenden, die bislang fiir 

 unangreifbar gehalten vverden durften." 



