702 POPULAR SCIEXCE MONTHLY. 



wash of the one in front. The advantage of this arrangement is 

 that a sufficient blade area is obtained to carry the thrust neces- 

 sary to drive the vessel with a lesser diameter of propeller, and so 

 permitting of a higher speed of revolution of the engines. 



The problem was complicated by the question of cavitation, 

 which, though previously anticipated, was first practically found to 

 exist by Mr. Thornycroft and Mr. Barnaby in 1894, and by them 

 it was experimentally determined that cavitation, or the hollowing 

 out of the water into vacuous spaces and vortices by the blades of 

 the propeller, commences to take place when the mean thrust pres- 

 sure on the projected area of the blades exceeds eleven pounds and 

 a quarter per square inch. This limit has since been corroborated 

 during the trials of the Turbinia. 



This phenomenon has also been further investigated in the case 

 of model propellers working in an oval tank of water, and to per- 

 mit of cavitation at more moderate speeds than would otherwise 

 have been necessary, the following arrangement was adopted: The 

 tank was closed, plate-glass windows being provided on each side, 

 through which the propeller could be observed, and the atmospheric 

 pressure was removed from the surface of the water by an air 

 pump; under this condition the only forces tending to prevent 

 cavitation were the small head of water above the propeller, and 

 capillary attraction. 



In the case of a propeller of two inches in diameter, cavitation 

 commenced at about twelve hundred revolutions, and became very 

 pronounced at fifteen hundred. Had the atmospheric pressure not 

 been removed, speeds of twelve thousand and fifteen thousand 

 respectively would have been necessary. 



Photographs were taken with a camera made for the purpose, 

 with a focal plane shutter giving an exposure of about one thou- 

 sandth of a second, the illumination being by sunlight concentrated 

 on the propeller from a twenty-four-inch concave mirror. 



Photographs were also taken by intermittent illumination of 

 the propeller from an arc lamp, the arrangement consisting of an 

 ordinary lantern condenser, which projected the beam on to a 

 small concave mirror, mounted on a prolongation of the propeller 

 shaft, the reflected beam being caught by a small stationary con- 

 cave mirror at a definite position in each revolution and reflected 

 on to the propeller. By this means the propeller was illuminated 

 in a definite position at each revolution, and to the eye it appeared 

 as stationary. The cavities about the blades could also be clearly 

 seen and traced, the photographs being taken with an ordinary 

 camera and about ten seconds' exposure. 



A series of experiments was also made with model propellers in 



