322 SECTIONAL TRANSACTIONS.— A. 



A position-vector P measured by light-signals using the kinematic time- 

 scale is equivalent to a position-vector II measured on the dynamic time- 

 scale, where P =U (tjt Q ). A problem first formulated explicitly by de Sitter 

 is thus solved, and his conjectural solution verified, by the process of the 

 identification of inductive laws of nature with laws reached deductively. 

 Reckoned on the kinematic time-scale, the age of the solar system is a small 

 multiple of io 9 years. But reckoned by dynamical events, such as the 

 number of swings of a macroscopic pendulum, the number of rotations of 

 the earth or the number of revolutions of a planet, the age is infinite. 

 The ' short ' and the ' long ' time-scales are thus in principle reconciled by 

 fundamental reasoning. 



Prof. W. H. McCrea. — R. A. Lyttletoris binary star hypothesis 

 concerning the origin of the Solar System. 



General Discussion on The evolution of the Solar System. 



Dr. H. Jeffreys, F.R.S. — A summing up of the discussion on the 

 evolution of the Solar System. 



Dr. W. Bowie. — The importance of isostasy in earth studies (12.20). 



During the past few decades isostasy has advanced from a purely theo- 

 retical concept to a very practical phase of earth sciences. By means of 

 geodetic data the ideas advanced in the last century by Pratt, Airy, Dutton 

 and others have been proved to be an actual physical characteristic of the 

 earth. While we cannot now say that isostasy has been proved beyond 

 question for the entire earth's crust, yet for those areas in which there are 

 abundant geodetic data and where the isostatic investigations have been 

 made, it has been found that, in its general aspects at least, the crust of the 

 earth is quite in conformity with the idea of isostasy. 



The isostatic reductions that have been made for deflection-of-the-vertical 

 and gravity stations give us a measure of the degree to which isostasy exists. 

 Necessarily in making the isostatic reductions of the geodetic data, a set 

 of very simple assumptions had to be made. The closeness with which 

 the theoretical and observed geodetic data can be brought into agreement 

 is the measure of the degree to which isostasy exists according to the 

 assumptions made. 



It is believed by many that the so-called isostatic anomalies for the de- 

 flection-of-the-vertical and gravity stations do not necessarily represent a 

 deviation of a large section of the earth's crust from normal mass. These 

 anomalies are due rather to a heterogeneous distribution of mass in the 

 outer portions of the earth's crust. It is this concept that is employed by 

 geophysicists and others in searching for buried geological structure that 

 has significance in locating ores, petroleum, natural gas and salt. It is only 

 when, for extensive areas, the gravity and deflection stations are very close 

 together, that one is justified in making any definite assumption as to the 

 dimensions of a block of the earth's crust that may be considered in equili- 

 brium independently of other portions of the crust. 



With isostasy proved, at least in its general aspects, we have a new start- 

 ing-point for geological research designed to unfold the history of the geo- 

 logical past and to discover the forces and processes that have been involved 

 in changes in the elevation of the earth's surface and the horizontal shifting 

 of rock. It is becoming evident to students of the earth that there must be 

 a wide application of physical and chemical methods in geological research. 



