In some frames used at the School of Mines for making large blue prints a similar device has been in use for several years. Instead, however, of the heavy and cumbrous back used by Mr. Parsons, a light, somewhat flexible back of one-quarter inch pine is employed, covered with heavy Canton flannel and several thicknesses of newspaper. The pressure is applied by light pressure strips of ash somewhat thicker at the middle than at the ends, which give a fairly uniform pressure across the width of the frame sufficient to hold the back firmly against the glass at all points. This system has been used with success for frames twenty-seven by forty-two inches, about half as large as the one described by Mr. Parsons. A frame of this size can be easily handled without mechanical aids. Care should be taken to avoid too great thickness and too much spring in the pressure strips, or the plate glass may be broken by excessive pressure. The strips used are about five-eighths of an inch thick at the middle, and taper to about three-eighths of an inch at the ends.

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