[Illustration: FIG. 111.--An effective arrangement of pulleys known as
block and tackle.]
163. Practical Application. In our childhood many of us saw with
wonder the appearance and disappearance of flags flying at the tops
of high masts, but observation soon taught us that the flags were
raised by pulleys. In tenements, where there is no yard for the family
washing, clothes often appear flapping in mid-air. This seems most
marvelous until we learn that the lines are pulled back and forth by
pulleys at the window and at a distant support. By means of pulleys,
awnings are raised and lowered, and the use of pulleys by furniture
movers, etc., is familiar to every wide-awake observer on the streets.
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