creditable to his genius. Nevertheless it had very considerable defects, and was finally found to be inefficient for the most important purposes to which he proposed applying it.

At the time of this invention, the mines in England had greatly increased in depth, and the process of draining them had become both expensive and difficult; so much so, that it was found in many instances that their produce did not cover the cost of working them. The drainage of these mines was the most important purpose to which Savery proposed to apply his steam engine.

It has been already stated that the pressure of the atmosphere amounts to about fifteen pounds on every square inch. Now, a column of water, whose base is one square inch, and whose height is thirty-four feet, weighs about fifteen pounds. If we suppose that a perfect vacuum were produced in the steam vessels v v' (fig. 12.) by condensation, the atmospheric pressure would fail to force up the water, if the height of the top of these vessels above the water to be raised exceeded thirty-four feet. It is plain, therefore, that the engine cannot be more than thirty-four feet above the water which it is intended to elevate. But in fact it cannot be so much; for the vacuum produced in the steam vessels v v′ is never perfect. Water, when not submitted to the pressure of the atmosphere, will vaporise at a very low temperature, as we shall hereafter explain; and it was found that a vapour possessing a considerable elasticity would, notwithstanding the condensation, remain in the vessels v v' and the pipe s, and would oppose the ascent of the water. In consequence of this, the engine could never be placed with practical advantage at a greater height than twenty-six feet above the level of the water to be raised.

(34.) When the water is elevated to the engine, and the steam vessels filled, if steam be introduced above the water in v, it must first balance the atmospheric pressure, before it can force the water through the valve B. Here, then, is a mechanical pressure of fifteen pounds per square inch expended, without any water being raised by it. If steam of twice that elastic force be used, it will elevate a column in F of thirty-four feet in height; and if steam of triple the force be used, it will raise a column of sixty-eight feet high,

which, added to twenty-six feet raised by the atmosphere, gives a total lift of ninety-four feet.

In effecting this, steam of a pressure equal to three times that of the atmosphere acts on the inner surface of the vessels v v'. One third of this bursting pressure is balanced by the pressure of the atmosphere on the external surface of the vessels; but an effective pressure of thirty pounds per square inch still remains, tending to burst the vessels. It was found that the apparatus could not be constructed to bear more than this with safety; and, therefore, in practice, the lift of such an engine was limited to about ninety perpendicular feet. In order to raise the water from the bottom of the mine by these engines, therefore, it was necessary to place one at every ninety feet of the depth; so that the water raised by one through the first ninety feet. should be received in a reservoir, from which it was to be elevated the next ninety feet by another, and so on.

Besides this, it was found that sufficient strength could not be given to those engines, if constructed upon a large scale. They were, therefore, necessarily very limited in their dimensions, and were incapable of raising the water with sufficient speed. Hence arose a necessity for several engines at each level, which greatly increased the expense.

(35.) These, however, were not the only defects of Savery's engines. The consumption of fuel was enormous, the proportion of heat wasted being much more than what was used in either forcing up the water, or producing a vacuum. This will be very easily understood by attending to the process of working the engine already described.

When the steam is first introduced from the boiler into the steam vessels v v', preparatory to the formation of a vacuum, it is necessary that it should heat these vessels up to the temperature of the steam itself; for until then the steam will be condensed the moment it enters the vessel by the cold surface. All this heat, therefore, spent in raising the temperature of the steam vessels is wasted. Again, when the water has ascended and filled the vessels v v', and steam is introduced to force this water through B B' into F, it is immediately condensed by the cold surface in v v', and does not

begin to act until a quantity of hot water, formed by condensed steam, is collected on the surface of the cold water which fills these vessels. Hence another source of the waste

of heat arises.

When the steam begins to act upon the surface of the water in v v', and to force it down, the cold surface of the vessels is gradually exposed to the steam, and must be heated while the steam continues its action; and when the water has been forced out of the vessel, the vessel itself has been heated to the temperature of the steam which fills it, all which heat is dissipated by the subsequent process of condensation. It must thus be evident that the steam used in forcing up the the water in F, and in producing a vacuum, bears a very small proportion indeed to what is consumed in heating the apparatus after condensation.

(36.) There is also another circumstance which increases the consumption of fuel. The water must be forced through B, not only against the atmospheric pressure, but also against a column of sixty-eight feet of water. Steam is therefore required of a pressure of forty-five pounds on the square inch. Consequently the water in the boiler must be boiled under this pressure. That this should take place, it is necessary that the water should be raised to a temperature considerably above 212°, even so high as 275°; and thus an increased heat must be given to the boiler. Independently of the other defects, this intense heat weakened and gradually destroyed the apparatus.

Savery was the first who suggested the method of expressing the power of an engine with reference to that of horses. In this comparison, however, he supposed each horse to work but eight hours a day, while the engine works for twenty-four hours. This method of expressing the power of steam engines will be explained hereafter.

(37.) The failure of the engines proposed by Captain Savery in the work of drainage, from the causes which have been just mentioned, and the increasing necessity for effecting this object, arising from the large property in mines which became every year unproductive by being flooded, stimulated the inge

nuity of mechanics to contrive some means of rendering those powers of steam exhibited in Savery's engine available.

Thomas Newcomen, the reputed inventor of the atmospheric engine, was an ironmonger, or, according to some, a blacksmith, in the town of Dartmouth in Devonshire. From his personal acquaintance and intercourse with Dr. Hooke, the celebrated natural philosopher, it is probable that he was a person of some education, and therefore likely to be above the position of a blacksmith. Being in the habit of visiting the tin mines in Cornwall, Newcomen became acquainted with the engine invented by Savery, and with the causes which led to its inefficiency for the purposes of drainage.

It has been stated that Papin, about the year 1690, proposed the construction of an engine working by the atmospheric pressure acting on one side of a piston against a vacuum produced by the condensation of steam on the other side. Papin was not conscious of the importance of this principle; for, so far from ever having attempted to apply it to practical purposes, he probably never constructed, even on a small scale, any machine illustrating it. On the contrary, he abandoned the project the moment he was informed of the principle and structure of the steam engine of Savery; and he then proposed an engine for raising water, acting by the expansive force of steam similar to Savery's, but abandoning the method of working by a vacuum.

This engine is described by Papin in a work published in 1707.

A (fig. 13.) is an oval boiler, having a safety-valve B, which limits the pressure of the steam. It is connected with a cylinder c, by a curved pipe having a stop-cock at D. A pipe with a stop-cock G opens from the top of the cylinder into the atmosphere, and a safety-valve F is placed upon the cylinder. A hollow copper piston H moves freely in the cylinder, and floats upon the water. o is a funnel with a valve L in the bottom, opening downwards, through which the cylinder c may be filled with water to the level of the top of the funnel. A close air-vessel communicates with the cylinder c by the curved tube, and has a valve K opening upwards. The force-pipe through which the water is raised communi

cates with the air-vessel I. If the cock D be shut, and the cock G opened, water poured into the funnel o will rise into

the cylinder c, the air which fills the cylinder escaping through the open pipe G. When the cylinder is thus filled with water, let the cock G be closed, and the cock D opened. The steam from the boiler, after heating the metal of the cylinder, will force the piston downwards, and drive the water through the curved tube into the vessel 1, from which its return is prevented by the valve K, which is closed by its weight. The air which filled the vessel I will then be compressed, and by its elasticity will drive a column of water up the pipe N. After the contents of the cylinder have been thus discharged it may be refilled in the same manner, and the process repeated.

It will be perceived that this project is nothing more than a reproduction of the engine of the Marquis of Worcester. In the preface to the work containing this description, Papin gives an extract from a letter addressed by him to Leibnitz in 1698, from which it appears that he had abandoned his idea of working the piston by the atmospheric pressure acting against a vacuum, considering it to be a contrivance inferior