510 THE LIFE OF JAMES D. FORBES. [CHAP. 



lucid and complete manner. It is only in recent years 

 that the vera causa has been pointed out which makes 

 Forbes' observational theory completely consistent with 

 recognized physical principles. tVe shall add a few 

 words on this subject after referring, more explicitly than 

 Forbes' modesty permitted him to do, to the immense 

 steps which he made towards the explanation of t!i 

 very singular phenomena ; steps whose immensity am 

 only now be thoroughly appreciated, when we sec round 

 us, and even occasionally among good mathematicians, 

 constant attempts to revive or to remodel one or other 

 of the old exploded theories. 



Forbes, as we have already said, did very completely 

 the Natural History of these enigmatic creatures, the 

 glaciers. His * viscous theory ' is rather the euuuciatinu 

 of a truth discovered by his own thoroughly scientific 

 observations and geodetic and other measurements, than 

 a physical theory. 



' Some confusion of ideas might have been avoided on 

 the part of writers who have professedly objected to 

 Forbes' theory, while really objecting only (and we 

 believe groundlessly) to his usage of the word viscosity, 

 if they had paused to consider that no one physical 

 explanation can hold for those several cases [mentioned 

 in the text] ; and that Forbes' theory is merely the proof 

 by observation that glaciers have the property that mud 

 (heterogeneous), mortar (heterogeneous), pitch (homo- 

 geneous), water (homogeneous), all have of changing 

 shape indefinitely and continuously under the action of 

 continued stress.' ' 



His detection of the * veined structure ' as something 

 essential to the understanding of glacier motion, his 

 tracing of its form and distribution in various parts of a 

 glacier, and his remarks on the production of the dirt- 

 bands, all show a very high order of inductive reasoning. 

 His observations as to pressure and distortion producing 

 the blue veins are very important. His explanation, 

 though incomplete, of the convexity of crevasses towards 

 1 Thomson and Tait's Natural Philosophy, 741 (footnote). 



