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POPULAR SCIENCE REVIEW. 
however, certain advantages peculiar to the turbine which do 
not belong to the water-wheel ; namely, that it can be made 
to work waterfalls of from 300 to 400 feet in height; and 
what is of considerable importance as regards efficiency is, that 
it works well in back-water. 
There are three different descriptions of turbines. 
1st. Turbines in which the water passes vertically through 
the wheel. 
Wheels of this class are composed of two annular cylinders, 
the upper fixed and the lower revolving on a vertical axis. 
The upper is fitted with guides to direct the water most 
efficiently against similarly curved vanes or buckets, turned 
in the opposite direction, in the lower wheel. The water 
passes from the reservoir or cistern, placed over the upper 
cylinder, vertically downwards, acting on the revolving wheel 
by pressure as it glides over the surface of the vanes. Burdin, 
about 1826, invented a turbine of this description {turbine a 
evacuation alternative), the efficiency of which was about 67 
per cent, of the theoretical fall. 
2nd. Turbines in which the water flows horizontally and 
outwards. 
In turbines of this class the revolving wheel is placed outside 
of the fixed wheel, so that the water directed by guide-plates 
on the inner wheel strikes the curved vanes of the outer wheel, 
and forces them round by pressure and reaction. The water 
is regulated by a cylindrical sluice fitting between the fixed and 
movable wheels. 
M. Fourneyron’s turbine is the chief example of this class. 
Its advantages, as stated by M. Poncelet in his report to the 
Academy of Sciences at Paris, are the high velocity at which 
it may be worked without reducing its useful effect, its small 
size, and lastly, its capability of working equally well under 
back-water. From the experiments of M. Morin, the co- 
efficient of useful effect * appears to range from 0 - 60 to 0’80. 
On the other hand, it has to the full the defects of this class 
of machines, requiring the utmost nicety of design and execu- 
tion, and being very susceptible of injury from small bodies 
carried into it by the water. It requires for its successful 
application both a large acquaintance with the principles of 
its construction and a considerable experience of its use. 
3rd. Turbines in which the water flows horizontally inwards; 
vortex wheels. 
We OAve the invention of this class of turbines to Professor 
James Thompson, G.E., of Belfast, and probably no turbines 
* The coefficient of useful effect is the per-centage of the power expended 
as compared with the work accomplished. 
