ENGINEERING BEFORE AND AFTER WAR 57 



discharge large volumes of high-pressure steam, which is 

 being utilized to generate about 10,000 horse-power by 

 turbines. At Solfatara, near Naples, a similar project is 

 on foot to supply power to the great works in the district. 

 It seems, indeed, probable that in volcanic regions a very 

 large amount of power may be, in the future, obtained di- 

 rectly or indirectly by boring into the earth, and that the 

 whole subject merits the most careful consideration. 



While on the subject of obtaining power, may I digress 

 for a few moments and describe an interesting phenome- 

 non of a somewhat converse nature viz. that of intense 

 pressure produced by moderate forces closing up cavities 

 in water. 



A Committee was appointed by the Admiralty in 1916 

 to investigate the cause of the rapid erosion of the propel- 

 lers of some of the ships doing arduous duties. This was 

 the first time that the problem had been systematically 

 considered. The Committee found that the erosion was 

 due to the intense blows struck upon the ^ blades of the 

 propellers by the nuclei of vacuous cavities closing up 

 against them. Though the pressure bringing the water to- 

 gether was only that of the atmosphere, yet it was proved 

 that at the nucleus 20,000 atmospheres might be pro- 

 duced. 



The phenomenon may be described as being analagous 

 to the well-known fact that nearly all the energy of the 

 arm that swings it is concentrated in the tag of a whip. 

 It was shown that when water flowed into a conical tube 

 which had been evacuated a pressure of over 140 tons 

 per square inch was recorded at the apex, which was ca- 

 pable of eroding brass, steel, and in time even the hardest 

 steel. The phenomenon may occur under some conditions 

 in rivers and waterfalls where the velocity exceeds 50 

 feet per second, and it is probably as great a source of 

 erosion as by the washing down of boulders and pebbles. 

 Then again, when waves beat on a rocky shore, under 

 some conditions, intense hydraulic pressures will occur, 



