"Another experiment may be, to take a glass tube... open at both ends, and fixed upright on legs, so that it need not be handled, for the hands might warm it: at the end of a quill, fasten five or six inches of the finest light filament of silk, so that it may be held either above the upper end of the tube or under the lower end, your warm hand being at a distance by the length of the quill. If there were any motion of air through the tube, it would manifest itself by its effect on the silk [Fig. I]; but if the tube and the air in it are of the same temperature with the surrounding air, there will be no such motion. ...Warm the tube, and you will find as long as it continues warm, a constant current of air entering below and passing up through it, till discharged at the top; because the warmth of the tube... rarefies that air, and makes it lighter than the air without, which therefore presses in below, forces it upwards, follows and takes its place, and is rarefied in its turn."