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 40. Therefore With every subject of instruction the question of its practical use must be raised, that nothing useless may be learned.

(ii) Whatever has been learned should be communicated by one pupil to the other, that no knowledge may remain unused. For in this sense only can we understand the saying: “Thy knowledge is of no avail if none other know that thou knowest.” No source of knowledge, therefore, should be opened, unless rivulets, flow from it. But of this will say more the following principle.

41. Nature becomes fruitful and strong through constant movement.

Thus, when a bird hatches eggs, it does not only warm them, but, in order that they may be warmed equally on all sides, it turns them round daily (this can be easily observed in the case of geese, hens, and doves, since these hatch their eggs under our very eyes). When the chicken has broken through the shell it exercises itself by moving its beak, its limbs, and its wings, by stretching itself and raising itself from the ground, and by repeated attempts to walk and to fly, until it is sufficiently strong to do so.

The more a tree is buffeted by the winds, the faster it grows and the deeper it drives its roots. Indeed it is healthy for all plants to be stimulated by rain, storms, hail, thunder, and lightning, and for this reason those localities that are greatly exposed to storms of wind and of rain ought to produce harder wood than others.

42. Imitation in the mechanical arts.—In the same way the builder leaves the wind and the sun to make his buildings dry and firm. The smith also, who wishes to harden and temper his iron, places it repeatedly in the fire and in water, and thus, by alternating cold and heat, and by repeatedly softening the metal, ultimately renders it hard and durable.