COMETS' TAILS 247 



numerous, where their relative velocities are greatest, and where, therefore, mutual 

 impacts, giving off incandescent gases, are the most frequent and the most violent. 

 This simple hypothesis explains easily many very striking facts about comets, such 

 as their sometimes appearing to send off in a few hours a tail many hundreds of 

 millions of miles in length. Wild notions of repulsive forces vastly more powerful 

 than the sun's gravity have been entertained ; bold speculations as to decomposition 

 (by solar light) of gaseous matter left behind it in space by the comet have also 

 been propounded ; but it would seem that the shower-of-stones hypothesis accounts 

 very simply for such an appearance. For, just as a distant flock of seabirds comes 

 suddenly into view as a dark line when the eye is brought by their evolutions into 

 the plane in which they fly, so the scattered masses which have lost velocity by 

 impact, while they formed part of the head, or those which have been quickened 

 by the same action, as well as those which lag behind the others in virtue of the 

 somewhat larger orbits which they describe, show themselves by reflected solar light 

 as a long bright streak whenever the earth moves into any tangent plane to the 

 surface in which they are for the time mainly gathered. 



A year later Tait found occasion, after the usual exposition of the 

 significance of the Conservation of Energy, to warn his students against 

 looseness of language. He illustrated his point by quoting from Bain's logic. 

 It would seem that he was put on the track of this book by W. Robertson 

 Smith, who had been carrying on his theological studies at the Free Church 

 College, Edinburgh, during 1868-70, and at the same time acting as Tail's 

 Assistant. In an undated letter, the main subject matter of which fixes the 

 date as April of 1870, Robertson Smith asked " Have you seen Bain's 

 Logic ? Full of rubbish about conservation of force, by which he means 

 momentum ! ! !" Tait's own lively criticisms of Bain's inaccuracies in scientific 

 statement will be found in Nature, Dec. i, 1870. 



It is the custom in the Scottish Universities for the Arts Professors in 

 rotation to address the graduates at the annual graduation ceremonial. In 

 the old days when the Arts Chairs were limited by statute to seven, each 

 Professor was called upon at intervals of about seven years to act as " Promoter" 

 and give an address. Tait was Promotor in 1866, 1874, 1881, and 1888. 

 Before his turn came round again, the new regulations had come into force, 

 and the Arts Faculty had been widened out to embrace nearly a dozen other 

 chairs in literature, history, education, science and art. 



Tait's first address on the value of the Edinburgh degree of M.A. was 

 published by the Senatus as a pamphlet. It contained a strong protest 

 against the proposition to amalgamate the Scottish Universities as one grand 

 National University with a central Examining Board. A quotation from 



