gutta-percha, papier maché, or tin, closed at one end and fitted with a cover at the other. They were covered with felt, drugget, or leather. The front ends of the carriers were provided with thick disks of drugget or leather fitting the tube loosely, and the opposite ends were surrounded with pieces of the same material attached to them like the leather of an ordinary lifting pump. In 1869 Messrs. Siemens Bros. received an order from the British government to install an experimental line of tubes between the central telegraph station and the general post-office. This was completed in 1870, and after a half-year’s test it was extended to Fleet Street, and finally to Charing Cross. The tubes were of iron, three inches in diameter, with flanged and bolted joints. It was found, after some experience, that there was no advantage in the circuit, so the up and down tubes were separated at Charing Cross Station and worked independently. =Recent Improvements in the London System.=—In 1870 Mr. J. W. Wilmot designed a double sluice-valve by means of which carriers could be despatched continuously without stopping the flow of air in the tubes. Mr. Wilmot further increased the working capacity of pneumatic tubes when, in 1880, he invented an intermediate automatic signaller, by means of which a carrier signals the passage of a given point on its journey, showing that the section of the tube traversed is clear, thus allowing a second carrier to be despatched before the first has reached its destination. [Illustration: FIG. 2. DIAGRAM ILLUSTRATING THE PNEUMATIC TUBE SYSTEM LONDON RADIAL SYSTEM. LONDON RADIAL SYSTEM.]