ABSTRACT
Every boy is interested in engines. Later on, we shall have occasion to examine the explanations which they give of its mechanism. But to start straight away with the analysis of these explanations would be to run the risk of misunderstanding them, since the child is speaking from memory and has nothing concrete before him. Let us begin, therefore, by questioning the children about a little toy engine which will be made to work in front of them. We have chosen one of those little engines which most boys play with, and of which the working can be almost entirely seen from outside. It consists of a vertical boiler, with a small spirit-lamp under it, from which, when it is lit, the steam escapes through a little pipe that can be seen from outside to run into the cylinder. The piston contained in the cylinder is invisible, but the connecting-rods come out at the bottom of the cylinder and work, by means of a metal bar, the big wheel outside on which a belt can be attached so as to utilise the force (see Fig. 6). The fire is, of course, lit in front of the child. Sometimes we tell him beforehand that there is water in the boiler, sometimes we let him find this out by himself. The latter is the best course to take, for we shall see that the younger children do not feel the need for any mediating factor between the fire and the wheel, whereas the older ones immediately look for such a factor. This is a valuable index to the mental orientation characteristic of each stage.
