The fourth screen (30-40 mesh) is made of iron wire and is held to the bottom support by six cleats which are punched from the body of the support. The bottom support is simply a flanged iron cover for the bottom of the canister. It is punched with 79 circular holes each 4 mm. in diameter and is painted on the outside to match the body of the canister. The screen and the inside of the bottom support or cover are coated with a red paint. AMERICAN MASK At the entrance of the United States into the war, three types of masks were available: the PH helmet, the British S. B. R. and the French M-2 masks. Experiments were made on all three of these types, and it was soon found that the S. B. R. offered the greatest possibilities, both as regards immediate protection and future development. During the eighteen months which were devoted to improvement of the American mask, the facepiece underwent a gradual evolution and the canister passed through types _A_ to _L_, with many special modifications for experimental purposes. The latest development consisted in an adaptation of the fighting mask to industrial purposes. For this reason a rather detailed description of the construction of the facepiece and of the canister of the respirator in use at the close of the war (R. F. K. type) may not be out of place. The mask now adopted as standard for the U. S. Army and Navy is known as the Model 1919 American mask, with 1920 model carrier, and will be described on page 225. [Illustration: FIG. 53.—Diagrammatic Sketch of Box Respirator Type Mask.] =Facepiece.= The facepiece of the R. F. K. type Box Respirator is made from a light weight cotton fabric coated with pure gum rubber, the finished fabric having a total thickness of approximately ¹/₁₆ inch. The fit of the facepiece is along two lines—first, across the forehead, approximately from temple to temple; second, from the same temporal points down the sides of the face just in front of the ears and under the chin as far back as does not interfere with the Adam’s apple. In securing this fit, the piece of stock for the facepiece is died out of the felt and pleated up around the edges to conform to this line. After this pleating operation, the edges of the fabric are stitched to a binding frame similar to a hat-band made up of felt or velveteen covered with rubberized fabric. All the stitching and joints in the facepiece are rendered gas-tight by cementing with rubber cement. This facepiece is made in five sizes ranging from No. 1 to No. 5, with a large majority of faces fitted by the three intermediate sizes, 2, 3, 4.