beneath it a shallow vessel, which takes up not only the bell-glass, but also a sufficient quantity of mercury to keep the gas imprisoned until the arrangements for the experiment are completed.

The extreme solubility of ammoniacal gas is, therefore, a property of which advantage may be taken for creating a vacuum, exactly as the same object is accomplished by the condensation of steam. As, on the other hand, the pressure which it is capable of exerting at given temperatures is much higher than that which steam affords at the same temperatures; and as, conversely, this gas requires a temperature considerably lower to produce a given pressure than is required by steam, it seems to possess a combination of properties favorable to the production of an economical motive power.

Ammonia, like several other of the gases called permanent, may be liquefied by cold and pressure. At a temperature of 38.5° C., it becomes liquid at the pressure of the atmosphere. At the boiling-point of water it requires more than 61 atmospheres of pressure to reduce it to liquefaction. The same effect is produced at the freezing-point of water by a pressure of 5 atmospheres, at 21° C. (70° F.) by a pressure of 9, and at 38° C. (100° F.) by a pressure of 14.

If a refrigerator could be created having a constant temperature of 0° C., or lower, liquid ammonia would furnish a motive power of great energy, without the use of any artificial heat. The heat necessary to its evaporation might be supplied by placing the vessel containing it in a water-bath, fed, at least during summer, from any natural stream. Such a condenser could not be economically maintained. A condenser at 21° C., however, and an artificial temperature in the boiler of 38° C., would furnish a differential pressure of 5 atmospheres, with a maximum pressure of 14. By carrying the heat as high as 50° C. (122° F.), a differential pressure of 11 atmospheres could be obtained, with an absolute pressure of 20.

These pressures are too high to be desirable or safe. Moreover, condensation is more easily effected by solution than by simple refrigeration, and hence, in the ammoniacal gas engines thus far constructed, the motive power has been derived, not from the liquefied gas, but from the aqueous solution. The gas is expelled from the solution by elevation of temperature. At 50° C. (122° F.) the pressure of the liberated gas is equal to that of the atmosphere. At 80° C. (176° F.) it amounts to 5 atmospheres, and at 100° C. (212° F.) to 7. At lower temperatures the gas is redissolved, and the pressure correspondingly reduced.

In the ammoniacal engine, therefore, the expulsion and resolution of the gas take the place of vaporization and condensation of vapor in the steam engine. The manner of operation of the two descriptions of machine is indeed so entirely similar, that but for the necessity of providing against the loss of the ammonia they might be used interchangeably. The ammonia engine can always be worked as a steam engine, and the steam engine can be driven by ammonia, provided the ammonia be permitted to

escape after use. The advantage of the one over the other results from the lower temperature required in the case of ammonia to produce a given pressure, or from the higher pressure obtainable at a given temperature. These circumstances are favorable to the economical action of the machine in two ways. In the first place they considerably diminish the great waste of heat which always takes place in the furnace of every engine driven by heat; the waste, that is, which occurs through the chimney without contributing in any manner to the operation of the machine. This waste will be necessarily greater in proportion as the fire is more strongly urged; and it will be necessary to urge the fire in proportion as the temperature is higher at which the boiler, or vessel containing the elastic medium which furnishes the power, has to be maintained. In the second place, that great loss of power to which the steam engine is subject, in consequence of the high temperature at which the steam is discharged into the air, or into a condenser, is very materially diminished in the engine driven by ammoniacal gas.

For instance, steam formed at the temperature of 150° C. (302° F.) has a pressure of nearly 5 atmospheres (4.8). I£ worked expansively, its pressure will fall to one atmosphere, and its temperature to 100° C. (212° F.), after an increase of volume as one to 4. If, now, it is discharged into a condenser, there is an abrupt fall of temperature of 50°, 60°, or 70°, without any corresponding advantage. If it is discharged into the air, this heat is just as much thrown away. In point of fact, when steam of 5 atmospheres is discharged into the air at the pressure of one, considerably more than half the power which it is theoretically capable of exerting is lost; and when, at the same pressure, it is discharged into a condenser, more than one quarter of the power is in like manner thrown away. And as the expansion given to steam is usually less than is here supposed, the loss habitually suffered is materially greater.

The ammoniacal solution affords a pressure of 5 atmospheres at 80° C. (176° F.), and in dilating to 4 times its bulk, if it were a perfectly dry gas, its temperature would fall below 0° C. But as some vapor of water necessarily accompanies it, this is condensed as the temperature falls, and its latent heat is liberated. The water formed by condensation dissolves also a portion of the gas, and this solution produces additional heat. In this manner an extreme depression of temperature is prevented, but it is practicable, at the same time, to maintain a lower temperature in the condenser than exists in that of the steam engine. It must be observed, however, that owing to the very low boiling-point of the solution it is not generally practicable to reduce the pressure in the condenser below half an atmosphere.

The advantages here attributed to ammoniacal gas belong also, more or less, to the vapors of many liquids more volatile than water; as, for instance, ether and chloroform. Engines have therefore been constructed in which these vapors have been employed to produce motion by being used alone, or in combination with steam. The economy of using the heat of exhaust

steam in vaporizing the more volatile liquid is obvious. But all these vapors are highly inflammable, and in mixture with atmospheric air they are explosive. The dangers attendant on their use are therefore very great. Ammonia is neither inflammable nor explosive, and if, by the rupture of a tube or other accident, the solution should be lost, the engine will still operate with water alone.

The action of ammonia upon brass is injurious; but it preserves iron from corrosion indefinitely. It contributes, therefore, materially to the durability of boilers. A steam engine may be converted into an ammonia engine by replacing with iron or steel the parts constructed of brass, and by modifying to some extent the apparatus of condensation.

ELECTRO-HEATING APPARATUS.

This invention, patented March 12, 1869, is based upon the well-known fact that electricity, in passing through a conductor of insufficient capacity (such, for instance, as a wire of very small diameter), evolves or develops heat. It is also well known that a wire of any great length, and of sufficiently small size to evolve considerable heat, will not conduct a strong current of electricity without difficulty and loss, and that as the wire becomes heated, its non-conductivity is increased, and that, in consequence, the heat becomes so great that the wire will be fused.

The object of the invention is to obviate this difficulty, by enabling a strong current of electricity to pass through a heat-evolving apparatus of any length; and to this end it consists in providing an electrical conducting coil, or chain, with intervals of small conducting power, in traversing which the electricity will be caused to evolve heat; and further, in interposing between said obstructing intervals free conductors of much larger size, which constitute reservoirs of electricity and radiators of heat, and will effectually obviate the difficulty experienced in a continuous length of conductor of insufficient capacity.

In this application of the invention, namely, for railway carriages or cars, it is proposed to employ magneto-electric machines, constructed especially for this purpose, for producing the requisite current, placed, if necessary, under the car, and to obtain the power to operate them from the axle of the car, thus taking advantage of a motive power which already exists, but of which, heretofore, no use has been made.

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A machine capable of heating to incandescence one foot of platinum wire one-tenth of an inch diameter, will heat 100 feet one-hundredth of an inch; 200 feet, two-hundredths of an inch, etc.; the law being that the lengths of the wires vary inversely in proportion to the squares of their diameters. Now, to reduce this to practice, it will be seen that a machine or battery of the power above referred to will heat a length of coil or chain, in which the aggregate length of the small wire of one-hundredth of an inch diameter, forming the obstructions, is 100 feet; and 200 feet, if their

diameters are reduced one half, etc. In other words, having a machine of a certain power, and a certain degree of heat is required, the diameters of the obstructing media may be reduced or increased in order to accommodate them to the power of the machine.

In order to warm an American car upon this plan, allowing for a tray placed in the floor of the car, in front of each seat, it is estimated it would require an entire length of the chain or coil of about 360 feet; and in which the obstructing media form an aggregate length of about 70 feet; so that to accomplish this it would require a machine to heat this latter number of feet of small wire. Although this may be a new application of electricity, and no machines can now be obtained already organized, and of sufficient power to be applied for this purpose, English electricians have made estimates of machines which come within all the requirements, as to power, space occupied, weight, power to operate them, etc., to make the invention practical and economical. Even with machines constructed for light-house purposes, 18 feet of number 20 iron wire can be melted instantly; and the fact is well known to electricians, if the same machine were organized for producing a current of quantity, the heating power would be greatly increased.

The inventor is not aware of any chemical battery by means of which this invention may be economically applied. In this case, the law of equivalents is in the way; and there must be a destruction of the battery corresponding to the amount of heat produced. In the course of time, however, chemical batteries inay be constructed so as to be applied advantageously, as, for instance, those having large metallic surfaces exposed to a weak chemical action; or earth currents may be accumulated and utilized for this purpose; but for the present he relies entirely upon the magneto-electric machine. Advantage may be taken of a train of cars going down grade, when usually the steam is cut off and the brakes put down, without taxing the locomotive at all; whereas, in case of combustion of coal, the loss is the same whether going up or down grade. Among some of the advantages claimed for this method of heating railroad cars are the following:

First, its economy; second, its safety; and third, its comfort. Concerning its economy, the trays may be constructed of hard wood, and covered by any metal, but copper would be best, on account of its absorbing heat more rapidly and retaining it longer. As regards the cost of magnet machines, this would be materially reduced if they were made by machinery and in large numbers, instead of by hand. There would be but little wear and tear of them except at certain points; and in case the magnets should in time become weakened, they could be easily taken apart and recharged. There being no strain or wear and tear upon the coil, being protected from injury by the plate covering it, and, besides, there being no possibility of its becoming oxidized by the degree of heat it would be subjected to, say 120 or 140 degrees, — it is supposed it would last for an indefinite period. It is to be borne

in mind, also, that by dispensing with stoves, 8 seats in each car are gained, and, consequently, a train of 7 cars would accommodate the same number of passengers, which, with stoves, would require 8 cars. In short, the percentage upon the original outlay would not compare to the annual expense of warming cars upon the plans now in use.

BRIDGES.

The East River Bridge.· -The plan of the East River Bridge, as proposed by Mr. Roebling, has met with the approval of the Board of U. S. Engineers, appointed to examine it, and of the Government, and has been fully adopted by the Board of Consulting Engineers, consisting of Horatio Allen, Wm. J. McAlpine, J. J. Serrell, Benj. H. Lathrop, James P. Kirkwood, and J. Dutton Steele, who have made to the Directors of the Bridge Company their final report, of which the following is the substance: The plans, including foundations, towers, and superstructure, have been laid before the board by Mr. Roebling at various times between February 16 and April 26, and from him they have received the fullest information touching all the details. Having completed the examination of the plans, and the investigation of the combinations and proportions proposed, the board deemed it an appropriate part of their duty to examine the structures of the same general character erected by Mr. Roebling across the Monongahela and Alleghany, at Pittsburgh, in 1846 and 1860; across the Niagara Falls in 1850, and across the Ohio, at Cincinnati, in 1860. They have thus had an opportunity of learning the successive steps in bridgebuilding, which, beginning with a span of 822 in 1854, and one of 1,057 feet in 1867, all standing this day, are a practical demonstration of the soundness of the principles and proportions on which these structures have been erected, and rendering unnecessary, at least for spans of 1,000 feet, any other demonstration, and affording the best source of information as to the practicability of taking another step in a span of 1,600 feet. The bridge proposed by Mr. Roebling, a steel wire cable suspension bridge, 1,600 feet between the towers, 135 feet above the water, will be, in the opinion of the board, a durable structure, of a strength sufficient to withstand six times the strain to which it can under any circumstances be subjected; that it will bear the action of the greatest storm of which we have any knowledge, and that the method of joining the parts cannot be surpassed for simplicity and security in the result.

In the United States, the most remarkable suspension bridges are Ellet's Wheeling bridge, over the Ohio, with a span of 1,010 feet; erected in 1848, and blown down in 1854. The Lewiston bridge, 7 miles below Niagara Falls, built by E. W. Serrel, spanned 1,010 feet. Roebling's bridge, at the Falls, spans 821 feet. McAlpine's new Niagara bridge has a span of 1,264 feet, and the proposed bridge to connect New York and Brooklyn is to have a span of 1,600 feet.