SCIENTIFIC SUMMARY. 
205 
each other in all directions, with here and there some minute scales of 
chlorite. These crystals, which readily depolarise polarised light, are 
nearly transparent ; hut the small amount of potassa, soda, and lime present 
in the rock, as shown by analysis, renders it improbable that so large a pro- 
portion of it can consist of any variety of felspar.” 
Professorship of Geology in King's College. — The chair has been given to 
Dr. P. Martin Duncan, F.R.S., Honorary Secretary to the Geological Society 
of London, and so well known for his researches on Fossil Corals. 
MECHANICAL SCIENCE. 
Farey and Donkin's Steam Engine. — The injurious effect of water in the 
cylinder of steam engines on their efficiency, is now pretty well understood. 
Water may exist in the cylinder, either as priming water carried over from 
the boiler, or as the result of condensation during the expansion, or from 
condensation on the surfaces of the cylinder, cooled during the expansion and 
exhaust, at the moment of admission of fresh steam. The two former causes 
of loss are best obviated by superheating the steam, the last by a steam 
jacket, which also tends, though less perfectly than a superheater, to prevent 
condensation during expansion. In all the best expansive engines now con- 
structed, these means of increasing the efficiency of the engine and reducing 
the consumption of coal are applied. But in the use of the steam jacket 
hitherto, while the sides and generally the ends of the cylinder have been 
heated, the piston has been left undefended against the cooling influence of 
the expanded steam. It has been proposed in France to convey steam to the 
interior of the piston, to maintain its temperature in the same way as the 
temperature of the cylinder sides is maintained by means of the jacket, but 
whether the proposal has been actually carried out in France we do not know. 
It has recently been accomplished in this country by Messrs. Bryan, Donkin, 
& Co. Further, in compound engines Messrs. Donkin have applied a steam 
superheater between the high pressure and low pressure cylinder. The 
steam lowered in temperature by a certain amount of expansion in the high- 
pressure cylinder is again raised to its first state by passing between plates 
heated by steam from the boiler, before entering the low-pressure cylinder. 
This mode of superheating presents certain practical advantages, and is said 
to have effected an economy of 7^ per cent. 
Mont Cenis Failway. — Engines have been constructed for the Mont Cenis 
Railway, with two outside cylinders driving the vertical wheels, and two 
inside cylinders driving the horizontal wheels which grip the mid rail. This 
system lias been adopted to obviate the ditficulties which attend the arrange- 
ment of toothed wheels first adopted, and the complicated link work subse- 
quently adopted to drive the four pairs of wheels of the mid-rail system, by 
means of a single pair of cylinders. The working weight of the new engines 
is 24^- tons. In regard to the Mont Cenis tunnel, 9,225 metres were com- 
pleted last year, and another year and a half is expected to suffice for its 
completion. 
Armour-plated Ships. — In a recent letter to Engineering , Mr. John Ericsson, 
