Several years later we began building these helicopteres for ourselves, making each one larger than that preceding. But, to our astonishment, we found that the larger the bat the less it flew. We did not know that a machine having only twice the linear dimensions of another would require eight times the power. We finally because discouraged, and returned to kite-flying, a sport to which we had devoted so much attention that we were regarded as experts. But as we became older we had to give up this fascinat