the total resistances, then a mechanical ventilator, utilizing 60 per cent. of the power employed, would, under the same conditions, have a duty of 11 lbs. of coal per horse-power on the air, being a saving of 64 per cent. At a depth of 550 yards to circulate the same volume, the duty of the furnace being 22 lbs. of coal, that of the mechanical ventilator would be 11 lbs., being a saving of 50 per cent."

While on this subject we may also state that a machine has been invented for the purpose of proper ventilation of mines, a description of which we take from the "London Mining Journal:"

"This subject has of late occupied the attention of mining and mechanical engineers, as well as that of others who have been startled into activity by the many appalling accidents which of late have occurred in consequence of the explosive gases being allowed to accumulate in mines. The question is, no doubt, one of great importance, and they who shall succeed, either by mechanical or other contrivance, in keeping up a constant supply of good wholesome air in all parts of a mine, will have conferred an incalculable boon upon mining science. Hitherto, little provision beyond the natural condition of things, or by adding a fire in the shaft or bottom of the shaft, has been adopted; and these, no doubt, in small shafts, and in mines of very limited extent, have been sufficient in determining a current by effecting thermometrical variations. There is a degree of uncertainty about it, however, in consequence of the varying conditions of the external atmosphere, changing as it does throughout the year. Hence machines have been invented for the purpose of blowing fresh air in and of exhausting foul air out of mines, some working by means of pumps, and others effecting the same object by means of centrifugal action. Mr. Lloyd, the able engineer of the Lilleshall Company, has been turning his attention to this, and he has succeeded in inventing a machine of ingenious construction, which the company has patented. The success of the plan appears to depend upon the peculiar construction and disposition of the fans, which beat the air out of the shaft, depending upon the well-known elasticity of the atmosphere to supply its place.

This he does by means of a centrifugal fan, driven by an engine. The one we saw was a beautifully executed model, with fan 18 inches in diameter and 6 inches wide over the blades, which, measured by the aerometer, produced exhaustion at the rate of 2,500 feet per minute, with a water-gauge of one-quarter inch. But the company are erecting a larger one, to be worked by a small horizontal, direct-action engine, which shall be described when in operation. It may be stated that the success which has hitherto attended the trials made surpasses all expectation, and the effects produced appear incredible, He first made a two-feet 3-inch fan, which exhausted 2,500 feet of air per minute; and another, with a 5-feet fan, one foot 10 inches broad, which exhausted 25,196 cubic feet per minute, with a water-gauge of two and three-eighths inches. Indeed, the effects were such as to be incredible to the inventor until after repeated measure

ments, in the course of which several aerometers were torn to pieces by the force of the current of air created."

THE MODERN ICE MACHINE.

The amount of ice produced by an ice machine, worked by means of an exhaust or condensing air-pump, driven by steampower, is easily determined, theoretically, from the_amount of coal burned in the furnace of the steam boiler. It has been proved that the combustion of one pound of anthracite coal produces, in round numbers, 14,000 units of heat, and that in order to freeze water of 72° F., it is necessary to abstract, besides 40° of sensible heat, 140° of latent heat together 180°— which, for one pound of water, is, of course, equivalent to 180 units of heat. As this number of units is the eightieth part of the 14,000 units produced by the combustion of one pound of coal, it is clear that the heat produced by the combustion of one ton of coal is equivalent to the heat to be abstracted from 80 tons of water of 72°, in order to change it to ice.

But in practice we find here exactly the same state of affairs as is the case with the steam engine. Theoretically, a steam engine ought to produce at least 700 units of force (foot-pounds) for every unit of heat consumed; in practice, good machinery only produces from about 70 to 100 foot-pounds, from about one-tenth to one-seventh part of the theoretical amount. In the best ice machines, thus far constructed, instead of freezing 80 tons of water for every ton of coal consumed, only from about 8 to 11 tons of ice are produced, also from one-tenth to one-seventh part of the theoretical amount; proving, thus, the remarkable fact, that in both the steam engine and the ice machine exactly the same relation exists between the theoretically calculated effects and the practical results.

As, however, all the best ice machines accomplish the conversion of the heat of the fuel into the freezing operation by the intervention of a steam engine, the fact that they practically produce only from one-tenth to one-seventh of the amount of the cold they theoretically should produce, is solely due to the other fact, that the steam engine, itself, practically produces only from one-tenth to one-seventh of the amount of power which would be strictly equivalent to the number of heat units consumed. It must not be lost sight of that it is only the power of the steam engine which generates the cold in the freezing machines, and that, therefore, improvements in the steam engine, which bring its practical results nearer to the theoretical standard, will at once exert their influence on the amount of ice the ice machines can produce, and, consequently, also on the cost of the ice manufactured in these machines.

Moreover, it appears that the kind of freezing machines in question, which convert power into cold, notwithstanding they are yet in their infancy, have already attained such a degree of excellence, that they are ahead of that class of machines which

convert heat into power, either by steam, hot air, or any other possible means, as it is proved that they produce the full theoretical equivalent of cold (negative heat) for the number of footpounds employed; namely, cooling one pound of water one degree for a power equivalent to 700 pounds descending one foot, which, expressed in the adopted scientific manner, is one unit of negative heat for every 700 foot-pounds consumed.

SOLAR HEAT AS A MOTIVE POWER.

M. Mouchot, in a contribution to the "Comptes Rendus," thus speaks of some of his results:

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According to my experiments, it is easy to collect, at a cheap rate, more than three-fifths of the solar heat arriving at the surface of the globe. The intensity of this calorific source, so feeble in appearance, was revealed by Pouillet, more than 30 years ago. At Paris, a surface of one square metre, normally exposed to the sun's rays, receives, at least, whatever may be the season, during the greater part of a fine day, 10 heat-units (calories) per minute. [The unit of heat adopted by most physicists is the quantity necessary to raise one pound of water from 0° to 1° C. We suppose M. Mouchot adopts the same standard.] To appreciate such an amount of heat, it is sufficient to observe that it will boil, in 10 minutes, one litre of water, taken at the temperature of melting ice, and it is almost equal to the theoretical power of a onehorse steam engine. Under the same conditions, a superficies of one 'are' (119.603 square yards) would receive, during 10 hours of insolation, as much heat as results from the combustion of 120 kilogrammes (321,507 pounds troy) of ordinary oil. These numbers are eloquent; they should, if not dispel, at least weaken the serious fears entertained by some, in consequence of the rapid exhaustion of coal mines, and the necessity of going to increasingly greater depths, disputing with the subterranean water this precious combustible. The intensity of the calorific radiation of the sun is, moreover, much less at Paris than in intertropical regions, or upon the elevated plains. It is, therefore, probable, that the invention of sun-receivers' will, some day, enable industry to establish works in the desert, where the sky remains very clear for a long time, just as the hydraulic engines have enabled them to be established by the side of water

courses.

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Although I have not been able to operate under very favorable circumstances, since my experiments have only been made with the sun of Alençon, Tours, and Paris, I proved, as far back as 1861, the possibility of maintaining a hot-air engine in motion, with the help of the sun's rays. More lately I have succeeded in boiling, tolerably quickly, several litres of water submitted to insolation. In short, having satisfied myself that it was sufficient to have a silver reflector, with a surface of one square metre, to vaporize, in 100 minutes, one litre of water (0.88 quarts), taken at the ordinary temperature, or, in other words, to produce 17

litres of vapor a minute, I tried to work a small steam engine by solar heat, and my efforts were crowned with success in June, 1866. In the mean time I have been able, by very simple appara tus, to obtain some remarkable effects from insolation, such as the distillation of alcohol, the fusion of sulphur, perfect cooking of meat, bread, etc. None of these experiments, particularly the application of the sun's heat to machinery, have been tried upon a sufficiently large scale. It would, therefore, be useful to repeat them in tropical countries, with sun-receivers' of suitable dimensions. We would measure the volume and the tension of steam produced in an hour by a given insolated surface, the pressure developed by the sun in a considerable mass of confined air, and the temperature which might be obtained by vast reflectors, formed of a framework of wood covered with plates of silver, etc."

IMPROVED MARINER'S COMPASS.

The Earl of Caithness is the inventor of a new mode of suspending ships' compasses, which for efficiency and simplicity is said to surpass anything yet produced. Instead of the two concentric brass rings having their axles at right angles, known as gimbals, Lord Caithness employs a pendulum and ball, which ball works in a socket in the centre of the bottom of the compass bowl. The compass works, therefore, on one bearing on the balland-socket principle, and thus maintains its parallelism with the horizon in the heaviest weather. If we may credit the published reports of the trials, the simplicity of this invention is not more striking than its efficiency. It is stated that it has already stood the most trying tests, and the oscillation of compasses to which it is applied, as compared with the oscillation of the gimbal compass, is as degrees to points.

TO MEASURE HEIGHTS.

A very compact and useful instrument, called the "apomecometer," that can be carried in the waistcoat-pocket, for ascertaining the vertical heights of towers, spires, and other buildings, has been invented in England. It cannot be better explained than by quoting the description given by Mr. Millar, the inventor:

The

apomecometer' is constructed in accordance with the principles which govern the sextant, namely, as the angles of incidence and reflection are always equal, the rays of an object being thrown on the plane of one mirror are from that reflected to the plane of another mirror, thereby making both extremes of the vertical height coincide exactly at the same point on the horizon glass; so that, by measuring the base line, we obtain a result equal to the altitude."

RESEARCHES ON MATERIALS FIT FOR RESISTING VERY HIGH TEMPERATURES.

M. Audouin. (Cosmos.) — While engaged with other studies on geology in the southern parts of France, the author found that between Tarascon and Antibes there exists a very valuable and extensive bed of bauxite (hydrate of alumina), which is occasionally applied for the manufacture of sulphate of alumina. This material has been applied, at the suggestion of Audouin, for the manufacture of crucibles and fire-bricks; and on having been tested in comparison with the best products of the kind from France, England, and Germany, it was found that even best firebricks might be melted in bauxite-made crucibles heated by mineral oils and a blast.

SILVER EXTRACTION

· ELECTRO-CHEMICAL TREATMENT.

To do away with the tedious and expensive process of amalgamation in the production of pure silver is a feat which Becquerel, Sen., of the French Academy of Sciences, asserts he has recently accomplished, after having experimented on this subject since the year 1835.

The experiment was tried successfully on 40,000 lbs. of silver ores from Peru, Mexico, and Chili, etc.

A powerful battery, with double liquid voltaic elements separated by porous diaphragms, was made to act on the prepared ore, from which the pure silver was thus obtained at once in a finely divided state and in a crystalline form. Messrs. Wolf and Pioche are at present, it is said, preparing for a trial of this system in California. Scientific American.

ON THE GLASS USED FOR LIGHT-HOUSES.

The special composition of the crown-glass used for the light apparatus for light-houses was, until quite recently, kept a secret by the manufacturers of Saint Gobain, in France, and some firms in Birmingham, which had the monopoly of this branch of trade.

From the researches of David M. Henderson, C.E., published in "Dingler's Journal," we are able to furnish the recipes for both of these.

The French glass is composed of:·

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