While a steamer is getting underway, the power of the engine is exerted, partly to overcome the inertia of the ship and partly to overcome the resistance of the water and air; but when the full speed is realized, the resistance from the inertia of the ship ceases, and the whole power of the engine is thenceforth exerted to overcome the resistance of the air and water—the vessel running by her own inertia. This principle is thus stated by W. James Grave-sande, in his great work entitled “The Mathematical Elements of Natural Philosophy,.”a translation of which, from the original Latin, was published in England in 1747:—.”A Ship drawn by a Rope Suffers Resistance from the Water: as long as this is less than that Pressure by which the Rope is drawn, the Celerity of the Ship is increased, and the Reaction, which i equal to the Action, as the Rope is equally stretched both Ways, is to be attributed in part to the Inertia of the Ship. When, the Celerity being increased, the Resistance of the Water has increased so much as alone to destroy the Action, whereby the Ship is drawn, it proceeds by its innate Force, with an equable Motion; two Pressures acting upon it, which mutually destroy one another ; as I observed before of the Carriage..” ___