It appeared with sufficient clearness in the immediately preceding Essay, that the intellect cannot be always on the stretch, nor the bow of the mind for ever bent. In the act of composition, unless where the province is of a very inferior kind, it is likely that not more than two or three hours at a time can be advantageously occupied. But in literary labour it will often occur, that, in addition to the hours expressly engaged in composition, much time may be required for the collecting materials, the collating of authorities, and the bringing together a variety of particulars, so as to sift from the mass those circumstances which may best conduce to the purpose of the writer. In all these preliminary and inferior enquiries it is less necessary that the mind should be perpetually awake and on the alert, than in the direct office of composition. The situation is considerably similar of the experimental philosopher, the man who by obstinate and unconquerable application resolves to wrest from nature her secrets, and apply them to the improvement of social life, or to the giving to the human mind a wider range or a more elevated sphere. A great portion of this employment consists more in the motion of the hands and the opportune glance of the eye, than in the labour of the head, and allows to the operator from time to time an interval of rest from the momentous efforts of invention and discovery, and the careful deduction of consequences in the points to be elucidated. There is a distinction, sufficiently familiar to all persons who occupy a portion of their time in reading, that is made between books of instruction, and books of amusement. From the student of mathematics or any of the higher departments of science, from the reader of books of investigation and argument, an active attention is demanded. Even in the perusal of the history of kingdoms and nations, or of certain memorable periods of public affairs, we can scarcely proceed with any satisfaction, unless in so far as we collect our thoughts, compare one part of the narrative with another, and hold the mind in a state of activity. We are obliged to reason while we read, and in some degree to construct a discourse of our own, at the same time that we follow the statements of the author before us. Unless we do this, the sense and spirit of what we read will be apt to slip from under our observation, and we shall by and by discover that we are putting together words and sounds only, when we purposed to store our minds with facts and reflections. We apprehended not the sense of the writer even when his pages were under our eye, and of consequence have nothing laid up in the memory after the hour of reading is completed.