Every great epoch has its master impulse, which acts as the precursor of a yet greater one to succeed it. A multitude of hearts may throb with ardent impatience, and myriads of hands may be ready to act, but not one profitable pulsation is there, nor an effective achievement, save as the actuating soul of the age shall animate and direct. All great revolutions in the intellectual world are marked by successive steps of generalization and transitions into wider realms through more expanded truths. We advance from the obscure to the obvious, from single facts to homogeneous combinations, and from particular doctrines to an all-comprehensive system. Nothing that does not relate to the perpetual progress of the great drama of divine Providence, and illustrate it, is admitted within our plan. With the whole field of human history before us, we are first to mark the most prominent features, and then trace whatever is subordinate and auxiliary. Four mighty landmarks rise most prominently to the view, around which are concentrated all the beneficent inventions and renowned names, universally admired by the civilized world. But, though supreme, these are not separate from inferior agents. True, the chief glory of an age, or people, seems to be the work of a few leading minds, while all others are transient actors on the stage. But each epoch, and all connected therewith, is a unit, indissolubly joined to its successors, in the formation of which it has contributed all the primary elements. Every subsequent act is the legitimate evolution of its predecessor, and from prelude to sequel, there is but one symmetrical development of an infinite plan. There may be deep and dark eddies in the stream, and even long reaches, wherein the current seems to assume a retrograde course, nevertheless its progress is not for a moment arrested, nor does it ever cease from innumerable tributaries evermore to augment its force. The spring-head we may not discern, but the main channel can be clearly traced through every clime, without meeting with whirlpools completely stationary, or depths too stagnant for some lofty use.