is one of those things which cannot be defined. We only know or become sensible of it through certain processes of nature which require it for their being carried on and perfected, and towards which it may therefore be said to bear a relation. We only appreciate it as a fact in the universal frame of things, when we are enabled by these means to measure it. Thus, the rotation of the earth on its axis, the process by which we obtain the alternation of day and night, takes a certain space of time. This, multiplied by 366, gives the time required for the revolution of

the earth around the sun, the process by which we enjoy the alternations of the seasons. The life of a well-constituted man will, under fair conditions, last during about seventy such spaces of time or years; very rarely to a hundred. The cluster of individuals termed a nation, or constituting a state, will pass through certain changes, inferring moral, social, and political improvement, in the course of still larger spaces of time; say several centuries: also certain processes of decay, requiring, perhaps, equal spaces of time. With such matters it is the province of history to deal; and actually from this source we learn pretty clearly what has been going on upon the surface of the earth during about four thousand years. We have also reason, however, to conclude, that our planet has existed for a prodigiously longer space of time than that. The