D. Brewster, J. D. Forbes, P. G. Tait 133 



Principalship of the United College of St. Andrews. His 

 demonstration of the polarization of heat by all the various 

 means by which ordinary light acquires that property, was 

 an experimental achievement of the highest rank, and was a 

 powerful link in the chain which connects the phenomena of 

 radiation. In another series of researches, Forbes appears 

 as one of the pioneers in the important but often neglected 

 field of Geo-physics. He was the first to conduct systematic 

 observations on the temperature of the earth, by inserting 

 thermometers reaching down to different depths beneath 

 the soil, in such a manner that they could be read off without 

 disturbing them. Such experiments allow us to measure the 

 thermal conductivity of the soil, and the loss of heat of the 

 earth through radiation. Later on he determined the thermal 

 conductivity of metals, and discovered that this conduct- 

 ivity diminished as the temperature increased. During a 

 number of visits to Switzerland he investigated the flow of 

 glaciers, and showed that the movement of the ice of glaciers 

 followed the laws of viscous bodies. The tremors of the 

 earth caused by earthquakes also occupied his attention, 

 and he constructed an instrument which was not sufficiently 

 sensitive, but must be considered as the forerunner of the 

 modern seismographs. 



Passing on to more recent times, the name of Peter Guthrie 

 Tait (1831-1900) has already been mentioned as belonging 

 to the Cambridge school of mathematics. The work of his 

 life was devoted to the Edinburgh University, where his 

 teaching of Natural Philosophy exerted a wholesome, though 

 perhaps restraining, influence on the many students who 

 passed through his hands. While he will be remembered 

 chiefly as a vigorous apostle of the doctrine of energy and a 

 forceful propagator of sound dynamical ideas, he made 

 substantial contributions to science, and the " Elements of 

 Natural Philosophy," written jointly by Thomson and Tait, 

 though ^never completed, is a monument " more permanent 

 than bronze." Associated with Tait as a prominent Univer- 

 sity teacher, the name of Crum Brown, Professor of Chemistry 

 between 1869 and 1908, will be remembered by many 

 students who passed through his hands. 



George Chrystal (1851-1911), another Cambridge man 



