��Popular Science Monthly
��of the negatives. That is, a positive from the green plate will show the leaves transparent, and the rose dense; while that from the "red" plate will show the leaves dense, and the rose transparent. Dyes used in this process affect only the dense places. It is obvious that if you want a red rose to be red in the resultant picture, you will have to color the positive from the "green" negative red, that being the only one showing the red rose as a dense area. Similarly, you will have to color the positive from the green negative "red," since in this case the leaves are dense. After dyeing them in this manner, the plates pass through a special process to eliminate the opaque black silver on the plates, leaving only the colored images. This process completed, the two plates are placed face to face and registered
��properly. Then you see the red rose in its proper place among the green leaves.
The next step is to cement these two positive emulsions together. This done, they are stripped from the glass and transferred to paper, canvas, ivory (in the case of a miniature), or any other backing. In their new positions they look not unlike an oil painting, especially when canvas is used as a mount.
For the sake of simplicity, the fore- going description of the red rose and green background referred only to these two colors. It is understood of course that almost any color which may have been present in the original object also appears in the finished picture. This is possible because red and green combined in different proportions by the process here used will give such desired colors.
��How a Steamer's Engine-Room Is Ventilated
��FRESH AIR DUCT-
��PHYSIOLOGISTS have shown in re- cent years that the chief effect of ventilation and open air treatment de- pends on the movement, temperature and moisture of the air, and less upon its chemical properties than was expected. For this reason the cooling of overheated engine-rooms, underground or underdeck, is best obtained, as engineers have dis- coN'ered, by flood- ing them with fresh air from outside under slight press- ure. This positive ventilation or a continuous change of air also removes a\\ noxious gases anfl smells emanat- ing from the oil and bilges.
The accompany- ing driiwing shows a transverse sec- tional view of the engine-room of the "Aquitania," with the recommended method for flood- ing the confined space with cool, fresh air under moderate pressure. The air is delivered
��OPEN MAIN HATCH
��A cross-seclion view of the "Aquitania,' showing ventilating arrangement
��into the engine-room by a large open fan placed at the junction of the lower ends of the air-shafts, so that the full volume of fresh air, equal in this instance to about one hundred and fifty thousand cubic feet per minute, is propelled into and properly distributed through the en- gine-room without lossfromdeliveryducts. When desirable, the air in the engine- room may be changed one hun- dred and twenty times an hourwith- out uncomfortable drafts. The cool air is drawn, not forc- ed, down from the ujipcr deck and delivered laterally by open fans jilaccd low down in the engine-room so as to Hood the whole space with air, the cooler incoming air falling towards the tliior, displacing the heated air and exjielling it up the main hatcli. Many transatlantic liners have the \enlilat- ing system illus- trated.