The method of graduating the indicator scales of the Frew pyrometer is worthy of special notice.  When the apparatus is fitted up, and before it is permanently fixed in position, the spiral heater is placed in cold water of known temperature, and the point noted at which the colored liquid stands in the indicator tube.  The water is then boiled, and the rise in the liquid in the tube is again noted.  Suppose, in the first instance, the cold water temperature to be 62 deg.  Fahr., and that, from this point up to 212 deg.  Fahr., the liquid to have risen 21/4 in. in the tube; this is equal to 11/2 in. per 100 deg.  Fahr., and from these data a scale is constructed, the correctness of which is easily verified by transferring the spiral heater into an air bath or oil of high boiling point, and then comparing the readings of the pyrometer scale with those of a mercurial thermometer placed alongside of the spiral heater.  By this means it can be clearly demonstrated that, up to the highest point to which it is safe to use a mercurial thermometer, the readings of the pyrometer scale and that of the thermometer are identical.

While this pyrometer is particularly valuable for indicating the temperature of hot blast stoves of every description, there are doubtless many uses that will suggest themselves to persons engaged in various industrial arts and manufactures.  The apparatus is neat and substantial in its parts, while it occupies very little space, is not at all liable to derangement, and is entirely automatic in its action.  A number of the instruments have been in continuous use at the Langloan Iron Works, with the most satisfactory results, for about eight months.  The temperatures they are graduated for vary according to the furnaces with which they are connected and the kind of work to which these are applied.—­Engineering.

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An exchange gives the following very simple way of avoiding the disagreeable smoke and gas which always pours into the room when a fire is lit in a stove, heater, or fireplace on a damp day:  Put in the wood and coal as usual; but before lighting them, ignite a handful of paper or shavings placed on top of the coal.  This produces a current of hot air in the chimney, which draws up the smoke and gas at once.

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[FROM PHOTOGRAPHISCHE CORRESPONDENZ.]

ORTHOCHROMATIC PLATES.

By CH SCOLIK.

Since the emulsion process has taken root, no improvement has awakened such a lively, steadily increasing interest as photography of colored objects in their correct tone proportions; a process which makes it possible to reproduce the warmer color-tones, particularly yellow, orange-red, and yellow-green, in their correct light value as they appear to the eye.