The importance of these details will be understood, when it is considered that the only limit to the attainment of speed by locomotive engines is the power to produce in a given time a certain quantity of steam. Each stroke of the piston causes one revolution of the wheels, and consumes two cylinders full of steam: consequently, a cylinder of steam corresponds to a certain number of feet of road travelled over: hence it is that the production of a rapid and abundant supply of heat, and the imparting of that heat quickly and effectually to the water, is the key to the solution of the problem to construct an engine capable of rapid motion. The method of subdividing the flue into tubes was carried much further by Mr. _Stephenson_ after the construction of the Rocket; and, indeed, the principle was so very obvious, that it is only surprising that, in the first instance, tubes of smaller diameter than 3 inches were not used. In engines since constructed, the number of tubes vary from 90 to 120, the diameter being reduced to 2 inches or less, and in some instances tubes have been introduced, even to the number of 150, of 1-1/2 inch diameter. In the Meteor, 20 square feet are exposed to radiation, and 139 to the contact of heated air; in the Arrow, 20 square feet to radiation, and 145 to the contact of heated air. The superior economy of fuel gained by this means will be apparent by inspecting the following table, which exhibits the consumption of fuel which was requisite to convey a ton weight a mile in each of four engines, expressing also the rate of the motion:-- +------------------+---------------+-----------------+ | |Average rate of|Consumption of | | Engines. | speed in miles|Coke in pounds | | | per hour. |per ton per mile.| +------------------+---------------+-----------------+ |No. 1. Rocket | 14 | 2·41 | | 2. Sanspareil | 15 | 2·47 | | 3. Phoenix | 12 | 1·42 | | 4. Arrow | 12 | 1·25 | +------------------+---------------+-----------------+