There are various regions of the earth, and these regions differ in temperature. The variation is more evident as we approach the poles; all bodies expand with heat and contract with cold; this is best measured in liquids and best of all in spirits; hence the thermometer. The wind strikes the face, then the air is a body, a fluid; we feel it though we cannot see it. I invert a glass in water; the water will not fill it unless you leave a passage for the escape of the air; so air is capable of resistance. Plunge the glass further in the water; the water will encroach on the air-space without filling it entirely; so air yields somewhat to pressure. A ball filled with compressed air bounces better than one filled with anything else; so air is elastic. Raise your arm horizontally from the water when you are lying in your bath; you will feel a terrible weight on it; so air is a heavy body. By establishing an equilibrium between air and other fluids its weight can be measured, hence the barometer, the siphon, the air-gun, and the air-pump. All the laws of statics and hydrostatics are discovered by such rough experiments. For none of these would I take the child into a physical cabinet; I dislike that array of instruments and apparatus. The scientific atmosphere destroys science. Either the child is frightened by these instruments or his attention, which should be fixed on their effects, is distracted by their appearance. We shall make all our apparatus ourselves, and I would not make it beforehand, but having caught a glimpse of the experiment by chance we mean to invent step by step an instrument for its verification. I would rather our apparatus was somewhat clumsy and imperfect, but our ideas clear as to what the apparatus ought to be, and the results to be obtained by means of it. For my first lesson in statics, instead of fetching a balance, I lay a stick across the back of a chair, I measure the two parts when it is balanced; add equal or unequal weights to either end; by pulling or pushing it as required, I find at last that equilibrium is the result of a reciprocal proportion between the amount of the weights and the length of the levers. Thus my little physicist is ready to rectify a balance before ever he sees one.