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given of the modus operandi : “ The line of communication 
cannot he cut, for the simple reason that the signalling 
takes place over the heads of the enemy, and the stations 
required are but few and far between. A 10-inch mirror — 
and this is the diameter of the ordinary field heliograph — is 
capable of reflecting the sun’s rays in the form of a bright 
spot, or flare, to a distance of fifty miles, the signal at this 
interval being recognisable without the aid of a glass. 
That is to say, two trained sappers, each provided with a 
mirror, can readily speak to one another, supposing the sun 
is shining, with an interval of fifty miles between them, 
provided their stations are sufficiently high and no rising 
ground intervene to stop the rays. 
Professor Reynolds, F.R.S., said that he had been able 
to get a barometer tube free from air by first washing the 
tube with water, and introducing the mercury while the 
tube was wet, and then leaving the tube in an inverted 
position for several days. The water absorbed the air, and 
floating up between the mercury and the glass left the tube 
dry, full of mercury, and free from air. He hoped at the 
next meeting to report some further experiments on the 
suspension of mercury by its cohesion in a tube 90 inches 
long. 
“On some Marine Fossil Shells in the Middle Coal Mea- 
sures of Lancashire.” 
The President said that Mr. George Wild, of the Bardsley 
Collieries, near Ashton-under- Lyne, had lately informed him 
of some very interesting fossil shells having been met with 
in sinking the deep pit at Ashton Moss, which has now 
reached the great depth of about 800 yards. 
In the Lancashire, Cheshire, Staffordshire, North Wales, 
Yorkshire, and Derbyshire Lower Coal Measures it has been 
long known that certain beds of marine shells of the genera 
