Page:The South Devon Atmospheric Railway.djvu/10

 July 1899. SOUTH DEVON ATMOSPHERIC BAILWAT. Fig. 2. Medhurst. Then he flays : " In many cases it will be practicable upon the same principle to form a tnbe so as to leave a continual communica- tion between the inside and the outside of it, without Buffering any part of the impelling air to escape, and by this means to impel a carriage along upon an iron road in the open air with equal Telocity, and in a great degree possessing the same advantages as in passing with-inside of the tube, with the additional satisfaction to passengers of being unconfined and in view of the country. Then he proposed to employ a 12-inch tube. It appears from a subsequent pamphlet that the mode by which he intended to make communication between the tube and the exterior was by a water seal, Fig. 2, but apparently he had discovered that the air-pressure of 3 lbs. and 6 oz. per square inch — that which he had assigned to a 12-inch tube — would be rather difficult to retain by a water seal, and he then proposed to use a 24-inch tube. In this later pamphlet (1827) he suggested a rectangular iron tube, with a wrought-iron or copper M semi-top," as he called it, riveted to the flange and lifted by the projection of a wheel attached to the piston, and thus admitting the protrusion of an arm to connect the piston inside the tube with the external carriage. There is not any description of how the valve was to be closed and sealed. The writer has come across the name of Lewis as having done something with atmospheric transmission of energy in 1817, but he has not been able to trace the particulars of that which Lewis did. In 1824, however, John Vallance took out his patent, No. 4905, so very well known to all who have interested themselves in this subject of transmission of energy by the pressure of the atmosphere.