satisfactorily proved that the spheroidal drop never touches the heated surface, but is separated from it by a considerable interval. To what, then, is this interval due? Let us quote the words of a clever writer to whom we are indebted for many of the facts contained in this chapter.
"At an early period of railway history, it was proposed by that original genius George Stephenson to substitute for ordinary steel springs, in the case of locomotives, springs of elastic steam. It was proposed to convey the steam into cylinders in which pistons should move steam-tight; these pistons supported by the steam beneath them, were to bear the weight of the locomotive. Now what the great engineer proposed for the locomotive, the spheroidal drop effects for itself—it is borne upon a cushion of its own steam. The surface must be hot enough to generate steam of sufficient tension to lift the drop. The body which bears the drop must be of such a nature as to yield up readily a supply of heat; for the drop evaporates and becomes gradually smaller, and to make good the heat absorbed by the vapour, the substance on which the drop rests must yield heat freely; in other words, it must be a good conductor of heat.
"It is to the escape of steam in regular pulses from beneath the drop that the beautiful figures which it sometimes exhibits are to be referred. By using a very flat basin over which the spheroidal drop spreads itself widely, we render it difficult for
