With respect to _the ease_ with which this method of communicating knowledge can be accomplished, we may remark in general, that when a principle has been explained, and has become familiar to the child, all the phenomena arising out of it, when pointed out, are readily perceived and retained upon the memory in connection with it. For example, by a knowledge of the principle which teaches that fluids press equally on all sides, when considered in connection with the weight of the atmosphere, a child, with very little trouble, would be put into the full possession of the cause of many facts in natural philosophy, exceedingly dissimilar in their appearance, but which are all mastered with ease and intelligence by a knowledge of this law. When the principle and its mode of working have been explained, the child is provided with a key, by which he may, in the exercise of his own powers, unlock one by one all the mysterious phenomena of the air and common pump, the cupping-glass, the barometer, the old steam and fire engine, the toy sucker and pop-gun, the walking of a fly on the ceiling, the ascent of smoke in the chimney, the sipping of tea from a cup, the sucking of a wound, and the true cause of the inspiration and expiration of the air in breathing. To teach these singly, would obviously be exceedingly troublesome to the teacher, and laborious for the child; but when thus linked together, as similar effects from the same cause, they are understood at once, and each of them helps to illustrate and explain all the others. They are received without confusion, and are remembered without difficulty. All this may in general be done even with children, as we shall immediately prove, by the method recommended above, of requiring, after the illustration of the principle, the lessons which it is calculated to teach. The results of this simple method of imitating Nature in one of the most valuable of her processes, have been found remarkably uniform and successful; and when it shall be regularly brought into operation in connection with the other parts of the system, it promises to be still more valuable and extensive. But even already, with all the disadvantages of time, place, and persons, the importance and efficiency of the exercise have been highly satisfactory. We shall shortly advert to a few instances of its success, which have been publicly exhibited and recorded.