356 RETROSPECT OP THE PRINCIPAL EPOCHS 



endeavours which the employment of the telescope called 

 forth. If we compare with the discovery of Ijhis optical 

 instrument, another great discovery belonging to a very 

 recent period, that of the voltaic pile, and the influence 

 which it has exercised on the ingenious electro-chemical 

 theory, on the production of the metals of the earths and 

 alkalies, and on the long sought discovery of electro- 

 magnetism; we arrive at a, series of phenomena called 

 forth at will, which in many directions enter deeply into 

 the knowledge of the dominion of the powers o nature ; but 

 which may rather seem to form a section in #ie history of 

 the Physical Sciences, than to. belong directly to the history 

 of the contemplation of the Cosmos. The multiplied con- 

 nections which link together the different branches of our 

 modern soien.ee, render it more difficujt to distinguish and 

 circumscribe them, We have even seen, most recently, 

 electro-magnetism acting upon the direction of &e pojarised 

 ray of light, and producing modifications like chemical 

 mixtures, Where, through the mental labours of the age, 

 the progressive development of knowledge is so rapid, it is 

 no less dangerous to attempt to lay a daring hand on the 

 intellectual process, and to paint that which is incessantly 

 advancing, as if the goal were already attained, than it is 

 for one sensible of his own limited powers, to venture to 

 pronounce on the relative importance of the honourable efforts 

 of those still living or recently departed, 

 . IB the historical considerations, describing the earlier 

 germs of our natural knowledge, I have, in almost all cases, 

 indicated the latest degree of development to which they 

 Uave attained, Tte ttH and last portion of my work IB 



