xiv.J FORBES' SCIENTIFIC WORK. 459 



from time to time to Forbes to be the correct one, and it 

 seems to us that a good deal of the hostile criticism 

 which has been lavished on his works is due to the 

 erroneous supposition that in his earlier papers he at- 

 tempted to propound a physical explanation ol glacier 

 motion. Another cause seems to lie in a mere verbal 

 quibble about the meanings of viscosity, plasticity, and 

 other similar terms. But the description of the ribboned, 

 or veined, structure, of the shearing motion throughout 

 the mass, and various other important points which must 

 have been seen, though not observed, by thousands before 

 Forbes, forms one most important part of his claim in 

 this matter. If mankind had till now neglected to notice 

 the annual rings of growth in trees, would not the natu- 

 t who first should call attention to their existence 

 importance deserve immense credit ? The veined 

 structure of a glacier is as profoundly and intimately 

 connected with its formation- and motions as are these 

 successive layers with the growth of a tree. There are 

 ral very singular peculiarities of this veined structure 

 and its relations of position to cracks and crevasses, 

 which appeared, especially as shown by Hopkins, to be 

 inconsistent with tin* well-known dynamics of stresses. 

 A very recent observation of Sir W. Thomson seems to 

 promise an explanation of this difficulty; for he has 

 shown that, in a viscous solid, continued shearing, 

 illel to one set of planes, gives rise to rupture along 

 one of th'-se planes. As the .subject of glaciers admits of 

 being more easily popularized than any of the other more 

 important scientific work of Forbes, and as it is not 

 difficult to select from his writings enough of simple but 

 comprehensive passages to enable us to let him tell the 

 story of his own doubts, of his determination to solve 

 tip in ly direct measurements, and finally of his un- 

 equivocal success, we shall devote to it another chapter, 

 instead of unduly expanding in favour of one particular 

 topic the present general sketch. 



The first experimental work of real novelty in which 

 Forbes successfully engaged was with reference to the 



