Page:The Steam Turbine.djvu/20

 being about 4,000 feet per second. Dr de Laval caused the steam to impinge on a paddle wheel made of the strongest steel, which revolved at the highest speed consistent with safety, or about half the velocity of a modern rifle bullet, for the centrifugal forces are enormous. Unfortunately, materials are not strong enough for the purpose, and the permissible speed of the wheel can only reach about two-thirds of that necessary for good economy, as I shall presently explain. Dr de Laval also introduced spiral helical gearing for reducing the enormous speed of rotation of his wheel (which needed to be kept of small diameter because of skin friction losses) to the ordinary speeds of things to be driven, and I shall allude to this gear later as a mechanism likely to play a very important part generally in future turbine developments.

In 1884 or four years previously, I dealt with the turbine problem in a different way. It seemed to me that moderate surface velocities and speeds of rotation were