of 70% has been obtained, and when a dynamo is driven directly

by them about 66 % of the hydraulic energy has been converted into electric energy. Pelton wheels are very sensitive to variation of load, and considerable trouble was experienced at first in securing adequate

Fig. 4 . — External View of Rigg's Water-Engine.

governing when they were used to generate electric energy; but this difficulty has been overcome, and they have been rendered most efficient machines for use with high falls, where ordinary turbines would be difficult to manage owing to the excessive speed at which they would run. In a small installation in the United States water is brought in a 36-in. pipe a distance of 1800 ft., and supplies six Pelton wheels each 28 in. in diameter, running at 135 revolutions a minute under a head of 130 ft. The total power developed is 600 H.P., and though the load factor varies very greatly in this case, the differential type of governor used secures perfect control of the running of the wheels.

Turbines. — The turbine has now become one of the most efficient of the prime movers employed by man, and in the United States of America and on the continent of Europe^ its use has enormously increased of recent years. Though no radical changes have been made in the design of turbines for some years, an immense amount of skiU and ingenuity has been shown in perfecting and improving details, and such machines of great size and power are now constantly being made, and give every satisfaction when in use. In the " Hercules " turbine, shown in fig. 6, the flow is whatis called mixed, that is, it is partly a radial inward and partly an axial flow machine. On entering, the water flows at first in a radial direction, and then gradually, as it passes through the wheel, it receives a downward component which becomes more and more important. Professor Thurston has published the results of a test

This and some of the other drawings have been taken from Blaine's Hydraulic Machinery.

The following statistics of turbine construction in Switzerlarid are taken from Schweizerische Bauzeilung (1901), p- 128, which, in the same volume at p. 53, contains a valuable article on the most important improvements in turbines and their regulation shown in the Paris Exhibition of 1901: — Fig. 5 .— Pelton Wheel. Period.