[21] We say living, because every attempt hitherto made to explain sensation, has been founded on certain appearances manifested in the dead subject.  By inspecting a dead carcass we shall never discover the principle of vision.  Yet, though there is no seeing in a dead eye, or in a camera obscura, optics deal exclusively with such inanimate materials; and hence the student who studies them will do well to remember, that optics are the science of vision, with the fact of vision left entirely out of the consideration.

* * * * *

ON THE BEST MEANS OF ESTABLISHING A COMMERCIAL INTERCOURSE BETWEEN THE ATLANTIC AND PACIFIC OCEANS.

To shorten the navigation between the eastern and western divisions of our globe, either by discovering a north-west passage into the Pacific, or opening a route across the American continent, with European philosophers and statesmen has for centuries been a favourite project, and yet in only one way has it been attempted.  Large sums of money were successively voted and expended, in endeavouring to penetrate through the Arctic sea; and such is the persevering enterprise of our mariners, that in all likelihood this gigantic task eventually will be accomplished:  but, even if it should, it is questionable whether a navigable opening in that direction would prove beneficial to commerce.  The floating ice with which those high latitudes are encumbered; the intricacy of the navigation; the cold and tempestuous weather generally prevailing there, and the difficulty of obtaining aid, in cases of shipwreck, must continue to deter the ordinary navigator from following that track.

Enquiry, therefore, naturally turns to the several points on the middle part of the American continent, where, with the aid of art, it is supposed that a communication across may be effected.  These are five in number, and the facilities for the undertaking which each affords, have been discussed by a few modern travellers, commencing with Humboldt.  On a close investigation into the subject, it will, however, appear evident, that although the cutting of a canal on some point or order, may be within the compass of human exertion, still the undertaking would require an enormous outlay of capital, besides many years to accomplish it; and even if it should be completed, the result could never answer the expectations formed upon this subject in Europe.  On all the points proposed, and more especially in reference to the long lines, the difficulty of rendering rivers navigable, which in the winter are swelled into impetuous torrents; the want of population along the greater part of the distances to be cut; the differences of elevation; and, above all, the shallowness of the water on all the extremities of the cuts projected, thus only affording admission to small vessels, are among the impediments which, for the time being at least, appear almost insuperable.