The high speeds--77 to 80 miles an hour--in view of which this stock has been constructed have, it will be seen, caused the elements relative to the capacity of the boiler and the heating surfaces to be developed as much as possible. It is in this, in fact, that one of the great difficulties of the problem lies, the practical limit of stability being fixed by the diameter of the driving wheels. Speed can only be obtained by an expenditure of steam which soon becomes such as rapidly to exhaust the engine unless the heating surface is very large. The tender, also fitted with wheels of 8 ft. 3 in. in diameter, offers no particular feature; it is simply arranged so as to carry the greatest quantity of coal and water. M. Estrade has also designed carriages. One has been constructed by MM. Reynaud, Bechade, Gire & Co., which has very few points in common with those in general use. Independently of the division of the compartments into two stories, wheels 8 ft. 3 in. in diameter are employed, and the double system of suspension adopted. Two axles, 16 ft. apart, support, by means of plate springs, an iron framing running from end to end over the whole length, its extremities being curved toward the ground. Each frame carries in its turn three other plate springs, to which the body is suspended by means of iron tie-rods serving to support it. This is then a double suspension, which at once appears to be very superior to the systems adopted up to the present time. The great diameter of the wheels has necessitated the division into two stories. The lower story is formed of three equal parts, lengthened toward the axles by narrow compartments, which can be utilized for luggage or converted into lavatories, etc. Above is one single compartment with a central passage, which is reached by staircases at the end. All the vehicles of the same train are to be united at this level by jointed platforms furnished with hand rails. It is sufficient to point out the general disposition, without entering into details which do not affect the system, and which must vary for the different classes and according to the requirements of the service. [Illustration: M. ESTRADE'S HIGH SPEED LOCOMOTIVE.] M. Nansouty draws a comparison between the diameters of the driving wheels and cylinders of the principal locomotives now in use and those of the Estrade engine as set forth in the following table. We only give the figures for coupled engines: TABLE II.