Page:Popular Science Monthly Volume 92.djvu/779

 Popular Science Monthly

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��Holding a Board with the Weight of a Newspaper

AVERY singular experiment can be carried out with a board about 3 ft. long and a piece of paper. A newspaper will do. The board is placed on a table with one-third of its length projecting over the edge. Cover the part of the board that is on the table with the news- paper, then ask one of your friends what will happen if you give the projecting end of the board a sharp blow downward with the fist. Most people will say that the board will spring off the table. But this is just what it will not do. Providing the blow be very sudden, the board will be immovable, no matter how hard the knock may be. The board should not be too wide so that a goodly portion of the paper will lay on the table top.

The explanation of this curious fact is to be found in the pressure of the air. It should be borne in mind that in the ordinary way the atmosphere is pressing

���The board and the paper as they are placed on the table top for the experiment

on every side of objects on this earth with an even weight. If the board on the table is given a sharp blow, there is no time for the air to rush beneath the paper to the underside of the board which is close to the table. As a consequence, there is a tremendous pressure on the upper part of the board, but none on the under part. This weight of air is so great that the board will break, if the blow is hard enough, before it will jump off the table. On the other hand a slow pres- sure, even wnth the little finger, will easily move the board. This is owing to the fact that the air has time, in these circum- stances, to pass underneath the board and newspaper — S. Leonard Bastin.

��A Homemade Wrench for Turning Small Inaccessible Nuts

THE little wrench shown in the il- lustration was made of an old foot pump handle. The parts being round it was only necessary to drill holes through

���Wrench made from a foot pump rod that will work like an ordinary monkey wrench

the movable jaw and the adjusting cli| then cut threads of the latter as well as oh the handle for making the wrench com- plete. Two nuts hold the adjusting piece on the outer jaw end. This wrench is an excellent one to use when making repairs on an automobile because it turns small nuts placed in places that are otherwise inaccessible. — Kenneth Whitney.

��Homemade Hectograph for Making Copies of Letters

A COPYING pad is indispensable to those who wish to make a limited number of copies of writings or drawings. One which is practical as well as in- expensive may be constructed in the following man- ner •, Procure 1 oz. of the best gelatine; cover it well with cold water, and allow it to stand over- night, care being taken to see that all of it has swollen. Heat 6

oz. of chemically pure glycerine over a salt water bath to a temperature not ex- ceeding 200 deg. F. The water that has not been absorbed by the gelatine should be poured off, and the gelatine added to the hot glycerine. The mixture obtained is heated for about an hour, and gently stirred occasionally, avoiding as much as possible any tendency of the fluid to froth or bubble. At this point add about a teaspoonful of oil of cloves as a preserva-

���The gelatine mixture placed in a shallow pan

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