Popular Mechanics, maaliskuu 1914
The bubbles in level glasses, set directly into the plaster-of-Paris bed, are difficult to see in some of the many positions the level is necessarily held while in use. The sketch shows an excellent way to set a glass so that the bubble will show up clear and plain in any position the body of the level may be held. Just before the glass is pressed into the plaster a thin strip of dark blue paper is wrapped halfway around the glass on the under side and pressed into place with the glass. - Contributed by F. W. Bently, Jr., Milwaykee, Wis.