A portion of putty is taken up on a knife, and inserted in the groove of the window sash. The pane of glass is then laid in the groove, and gently pressed down in every part, so as to lie on the putty. As the sheets of glass are never perfectly flat, it is a rule among glaziers to let the _concave_ side of a pane be within doors and the convex side without. After the glass is laid in, the edge is carefully coated with putty, to the extent of about an eighth of an inch: if this be carefully done, it is sufficient to secure the glass in its place, without presenting an unsightly appearance from the interior of a room. The opposite side of the glass now requires a little attention, since the bed of putty originally laid in the groove has been partially squeezed out by the pane of glass: a little trimming and finishing are all that are required in this matter. When a broken pane is to be replaced in a window, it is done generally without taking out the sash; but in the case of glazing the sashes of a new house, such as we have been supposing, it is done before the sashes are fitted into their places. If sashes are glazed with _plate_ instead of _crown_ glass, the only difference in the glazier’s method of proceeding is, that the pane being heavier, must be fixed in with greater attention to security. Sometimes a small beading or fillet of wood is used instead of putty, in which case it is either nailed or screwed to the sash. Where skylights are used instead of windows, a different plan must be observed, since there are no cross bars to the sashes. In this case the squares of glass are fixed in somewhat in the way adopted in slating a roof, that is, the lower pieces are puttied in first, and the upper ones are lapped over them, so that each pane projects about three-quarters of an inch over the one next below it. This is to effect two objects,--to prevent the necessity of puttying the joints, and to exclude rain. _Ground_, _fluted_, _painted_, _stained_, and _embossed_ glass, are occasionally employed for windows. These need not be noticed, since the processes by which they are fluted, stained, &c., would carry us to details of too extensive a nature. So far as the glazier is concerned, rather more care and delicacy are required in proportion as the kind of glass employed is more costly or more ornamental. In some of the better kinds of houses, rooms are provided with double windows, separated a few inches from one another. The object of this is, to prevent the room from being affected by rigorous cold from without; for a mass of air _when stationary_, conducts heat very slowly; the stratum of air between the two windows, therefore,--being stationary,--is slow to conduct the cold from without, or, more correctly, to conduct the warmth from within. Sheet Lead for Roofs and Cisterns.