order to hoist it or lower it. Although that is an extremely large
apparatus, yet by means of the above device it was worked easily and
rapidly, and gave every satisfaction.

The solution used was of the same proportions as had been adopted in
the other engineering offices of the road:

Citrate iron and ammonium 1-7/8 oz.
Red prussiate potash (C.P.) 1-1/4 oz.

Dissolve separately in 4 oz. distilled water each, and mix when ready
to use. But by putting mixture in dark bottle, and that in a tight box
impervious to light, it can be kept two or three weeks.

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.