664 
POPULAE SCIENCE EEVIEW. 
Pressure and Density of Saturated Steam. — Mr. Thomas R. Edmonds, B.A., 
has commmiicated to the Philosophical Magazine two interesting papers on 
the laws of density and pressure in saturated steam, giving new formula? of 
great simplicity, and closely agreeing with experiment. The papers are too 
mathematical to he given in abstract here. 
Gun-Cotton Engine. — M. Jules Gros has invented an engine which is 
moved by the action of compressed air, obtained by the explosion of gun- 
cotton in a strong receiver. 
Screw Propeller. — Some remarkable experiments have been carried out 
upon a steamer belonging to Messrs. Rennie, with the ordinary Admiralty 
Mangin screw and with a Griffith’s screw, fitted Avith an addition, the inven- 
tion of Mr. Riggs. This addition consists of a “ boss screwf resembling an 
ordinary many-bladed screw, placed behind the revolving screw, and attached 
to the stern-post. This supplementary screw propeller is fixed, and has its 
blades set at precisely the opposite angle to those of the revolving screw. 
According to the theory of the inventor, the action of this screw is as 
follows : — At the instant of the water being thrown off bj’’ the propeller, it 
is arrested and caused to deviate by the fixed blades as it impinges on them. 
Hence the water, instead of assuming the form of a cone, directed upwards, 
and deflected to the right hand or the left, according to the twist of the 
screw, is sent straight back in a column parallel with the ship’s keel, 
whereby power is saved, vibration arrested, and the steering improved. It 
is probable that this theory will require revision, but the experiments given 
below show that the invention has some real advantages : — 
Speed in knots Revolutions of screw 
per hour. per minute. 
Admiralty Screw 5 ‘871 227 
Griffith’s and Rigg’s Screw 7*574 184 
New Pump. — Mr. Wilson has invented an ingenious form of pump, in 
which valves are entirely dispensed with. It consists of a cylinder oscillating 
in a tank of water, on trunnions near its centre. The oscillation is caused 
by the revolution of the crank simultaneously moving the piston. As the 
cylinder oscillates, it brings the ports on its side alternately opposite to open- 
ings from the supply-tank and into the discharge-chamber. 
