Page:Popular Science Monthly Volume 64.djvu/143

Rh than this one have since been built and prove beyond a question that a practical, efficient and powerful method of combination of small forces has been discovered.

Dr. Bell has been building during the past summer thousands of tetrahedral cells varying in size from 25 cm. to 1 meter. Some of them are covered with light red silk weighing about 40 grams to the square meter and others with nainsook, very fine cotton, about as light as the silk. Some of the earlier cells were covered with cheesecloth, but the cheesecloth weighed so much—over 100 grams to the square meter—that Dr. Bell has discarded it. The framework of the cells is usually of black spruce, which is light and strong.

To make a tetrahedral cell, take six sticks of equal length and place three of them on a table so as to make an equilateral triangle. Erect one of the three remaining sticks at each corner of the triangle and bring the tops together. It is the old-fashioned puzzle of making four triangles with six matches. Then cover any two sides and you have a tetrahedral winged cell.

A number of cells outlined against the sky look like a flock of birds; for instance look at Fig. 18; the wings of a cell are also like a bird's wings in that they are not rigid like a board; the silk covering yields somewhat to the pressure of the wind as the feathers of a bird's wing.

Hundreds of tetrahedral cells are now being made in which the frame consists of hollow aluminum tubing. The aluminum weighs very little more than the spruce wood hitherto employed and gives much greater strength to the frame.

Using these cells just as a mason uses bricks to build houses of many forms, he has been constructing kites of every shape that a fertile brain could devise. Steadiness in the air and lifting power have been the main object in all. Some of his combinations are gigantic, exceeding twenty-five feet in length and twelve and fifteen feet in height and width, but in spite