over the whole surface and always kept it in position.  The frame was run in and out of the printing room on a little railway on which it rested on four grooved brass sheaves, one pair being at one end, while the other was just beyond the center, so the frame could be revolved in direction of its length without trouble.  In order to raise the heavy back, I had a pulley-wheel fastened to the ceiling, through which a rope passed, with a ring that could be attached to a corresponding hook at the side of the back, in 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.

The formulae for the solution given by Whittaker, Laudy, and Parsons are practically identical so far as the proportions of citrate of iron and ammonia and of red prussiate of potash, 3 of the former to 2 of the latter, but differ in the amount of water.  Laudy’s formula calls for about 5 parts of water to 1 of the salts, Whittaker’s for 4 parts, and Parson’s for a little more than 2 parts.  The stronger the solution the longer the exposure required.  With very strong solutions a large portion of the Prussian blue formed comes off in the washwater, and when printing from glass negatives the fine lines and lighter tints are apt to suffer.  The blue color, however, will be deep and the whites clear.  With weak solutions the blues will be fainter and the whites bluish.  Heavily sized paper gives the best results.  The addition of a little mucilage to the solution is sometimes an advantage, producing the same results as strength of solution, by increasing the amount adhering to the paper.  With paper deficient in sizing the mucilage also makes the whites clearer.—­H.S.M., Sch. of M. Quarterly.