temperature.  It will readily be seen that the strain brought upon the seams along the bottom is something enormous, and we can understand why it is that many boilers of this class rupture their girth seams while being filled up for the night after the engine has been shut down.  To most persons who have but a slight knowledge of the matter, we fancy it would be a surprise to see the persistence with which cold water will “hug” the bottom of a boiler under such circumstances.  We have seen boilers when the fire has been drawn, and cold water pumped in to cool them off, so cold on the bottom that they felt cold to the touch, and must consequently have had a temperature considerably below 100 deg.  Fahr., while the water on top, above the tubes, was sufficiently hot to scald; and they will remain in such a condition for hours.

[Illustration:  FIG. 2.]

The only thing to be done, where feed connections are made in the manner described, is to change them, and by changing them at once much trouble, or even a disastrous explosion, may be avoided.  Put the feedpipe in through the front head, at the point marked p in Fig. 1, drill and tap a hole the proper size for the feed pipe, cut a long thread on the end of the pipe, and screw the pipe through the head, letting it project through on the inside far enough to put on a coupling, then screw into the coupling a piece of pipe not less than eight or ten feet long, letting it run horizontally toward the back end of the boiler, the whole arrangement being only from 3 to 4 inches below the water line of the boiler, and hot or cold water may be fed indifferently, without fear of danger from ruptured plates or leaky seams.  In short, put in a “top feed,” and avoid further trouble.—­The Locomotive.

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[MICROSCOPICAL JOURNAL.]

IRON PRINTING AND MICROSCOPIC PHOTOGRAPHY.

By C.M.  VORCE, F.R.M.S.

I. FORMULAS FOR PRINTING SOLUTIONS.

Blue Prints.—­The best formula for this process, of many that I have tried, is that furnished by Prof.  C.H.  Kain, of Camden, N.J., in which the quantity of ammonio-citrate of iron is exactly double that of the red prussiate of potash, and the solutions strong.  This gives strong prints of a bright dark blue, and prints very quickly in clear sunlight.

Dissolve six grains of red prussiate of potash in one drm. of distilled water; in another drm. of distilled water dissolve twelve grains of ammonio-citrate of iron.  Mix the two solutions in a cup or saucer, and at once brush over the surface of clean strong paper.  Cover the surface thoroughly, but apply no more than the paper will take up at once; it should become limp and moist, but not wet.  The above quantity of solution, two drms., will suffice to sensitize ten square feet of paper, or three sheets of the “regular” size of plain paper, 18x22.  As fast as the sheets are washed over with the solution, hang them up to dry by one corner.  The surplus fluid will collect in a drop at the lower corner, and can be blotted off.