PROFESSOR CHRYSTAL 87 



cellar which was used originally as a store room but which after 1892 was 

 equipped as a research room for my work on magnetic strains. 



The old anatomical theatre was adapted to the purposes of the Mathe- 

 matical Department under Professor Chrystal, who began his Edinburgh 

 professoriate in the same transition year 1879-80. These changes brought the 

 Mathematical and Natural Philosophy Departments into closer contiguity; but, 

 what was of still greater importance, Professor Tait found in his new colleague 

 an enthusiastic experimentalist, who from 1880 to about 1886 passed the 

 summer sessions in the Physical Laboratory, exercising a stimulating influence 

 upon many of the students who were devoting themselves to practical physics. 

 Chrystal had just written the articles on Electricity and Magnetism for the 

 Encyclopaedia Britannica and was thoroughly posted on all the recent work 

 in these rapidly developing branches of physics. In carrying out his important 

 researches on the differential telephone and the measurements of inductances, 

 he had all the facilities of the laboratory placed at his disposal ; and both 

 directly and indirectly he gave many a hint to the students who were able 

 to take advantage of their opportunities. When I left for Japan in 1883 

 Chrystal was almost as strong an influence in the Laboratory as Tait himself ; 

 but after a few years the increasing duties of his own chair and the fact that 

 he found himself to be appropriating more and more of the really serviceable 

 apparatus for his own experiments obliged him to relinquish experimenting 

 for some time. 



When, mainly through the exertions of Dr Buchan, the Ben Nevis 

 Observatory was started in 1883, attention was drawn to the difficulty of 

 measuring humidities of the atmosphere under the conditions which frequently 

 existed on the top of the mountain. Both Chrystal and Tait suggested forms 

 of instrument for the purpose. Chrystal's was on the principle of Dine's 

 hygrometer, the nickel plated copper box, into which the thermometer bulb 

 was inserted, being supplied by means of a double tap arrangement with warm 

 or cold water at will. The temperature was adjusted until a film began to 

 form on the box. Tail's instrument was constructed on a totally different 

 principle, that of the atmometer. The following is Tail's description from 

 his paper of February 16, 1885 (Proc, R. S. E. Vol. xin, p. 116). 



" The atmometer is merely a hollow ball of unglazed clay, to which a glass tube 

 is luted. The whole is filled with boiled water and inverted so that the open end 

 of the tube stands in a dish of mercury. The water evaporates from the outer 

 surface of the clay (at a rate depending partly on the temperature, partly on the 



