209 



lates contain results of experience. Accordingly, the multifarious 

 proofs of the laws of aggregation, in the case of pressures, are not 

 the mathematical playthings which they are often supposed to be 

 from their grounds being insufficiently stated. If to the postulates 

 necessary to make the law of aggregation a consequence, be added 

 the following, " the velocity due to the aggregate of pressures in one 

 given direction is the aggregate of the velocities due to the pressures 

 taken separately," it follows, as a mathematical consequence, that 

 the pressure varies as the velocity created by it in a given time. 

 This great law of dynamics, therefore, is not fundamentally distinct 

 from the laws of aggregation, but follows from them with the addition 

 of a postulate more simple than itself. 



A proof was also given by Professor Stokes, of the theorem that 

 " Every Equation has a Root." 



May 2, 1859. 



Professor Miller made a communication " On the employment 

 of the Gnomonic Projection of the Sphere in Crystallography." 



May 16, 1859. 



Mr. Hopkins gave a lecture " On Glacial Theories." 

 De Saussure attributed the motion of glaciers to their sliding over 

 the bottoms of the valleys in which they exist, but did not appear 

 to have made any observations on the change of form to which the 

 glacial masses may be subject during their motion. No advance 

 was made in our knowledge of glaciers for many years after De 

 Saussure's death, till certain Swiss observers, Charpentier and others, 

 rather more than twenty years ago, added much to our knowledge 

 of the subject. Among these glacial observers, M. Agassiz, with 

 characteristic zeal and activity, soon afterwards took a prominent 

 position. He and those who were associated with him, or had pre- 

 ceded him, brought forward incontestible evidence of the former ex- 

 tension of glaciers in the Alps, and of their efficiency in the trans- 

 port of enormous angular fragments of rock from their original sites 

 to other localities, not only in the same Alpine valley, but even on 

 the flanks of the Jura on the opposite side of the great central valley 

 of Switzerland. These were the great facts which bore upon specu- 

 lative geology. Glaciers were still engaged in the work of trans- 

 port, and they had been so on a much larger scale at some former 

 epoch ; the masses of ice of which they were composed had been 

 then of much larger dimensions, and consequently the mean climate 

 of Western Europe must have been at that period considerably lower 

 than at present. No one was so active as M. Agassiz in pressing 

 these facts on the notice of geologists, or insisted more strongly on 

 their geological importance ; and by the energy of his own character, 

 his great reputation, and extensive personal acquaintance with men 



