a contrivance of great utility for enabling an observer to determine the altitude of a heavenly body, or of a terrestrial object, above the horizon of any place, when the sensible horizon is ill defined. The surface of a fluid not easily disturbed by the air, such as quicksilver, or some viscid, opaque fluid, is usually employed for this purpose, as they will adjust themselves to a plane parallel to the rational horizon. To prevent the influence of winds in the open air, the surface is usually covered by a plate of ground-glass with parallel surfaces. In fixed observations this is not necessary; and the mercury is contained in an oblong trough: for locomotive observations, a cup of three inches in diameter is the containing vessel. But as carrying about mercury is inconvenient, some have employed polished metallic or glass mirrors, adjusted by screws at the corners, and a spirit-level, to horizontality; though this is less accurate than the fluid surface. The surface of the quicksilver, or mirror so adjusted, is a plane touching the surface of the earth where the observation is made, and parallel to the rational horizon; therefore, a ray of light passing from the object to the surface of the instrument, forms an angle with that surface equal to the angular elevation of that object above the true horizon of the place, when it is corrected for parallax and refraction. (T. S. T.) The term artillery, in modern warfare, is applied, in a special sense, to those projectile machines which, from their magnitude and weight, cannot be made the personal weapons of a soldier; and, in a general sense, to the officers and men, that is to say, to the personnel as well as to the materiel of that branch of the army to which the care and management of such machines have been confided. The origin of artillery, in the modern and special meaning of the term, may be simply ascribed to the substitution of a chemical for a mechanical agency in the projection of missiles of war; and the great superiority of modern artillery depends on the power of conferring great velocities on the bodies to be moved, by the sudden development of gaseous bodies from materials previously in a solid form. Notwithstanding the great improvements in modern machinery, it may be safely assumed, that by no application of the property of elasticity in metallic, wooden, or other springs, could a convenient machine have been formed to propel a 32-pound shot with a velocity of 1600 feet per second, although it is highly probable that one might have been constructed to propel very large masses with a low velocity, or to produce great momenta, as was in fact the case with some of the projectile machines of the ancients. Gunpowder, or the chemical agent now used in propelling warlike missiles, is a compound of nitre, charcoal, and sulphur, three substances which, under ordinary circumstances, retain the solid form, although intimately mixed together and in contact with each other. By the application of heat amounting to about 570° of Fahrenheit, by the electric spark, or by the shock between the bodies, and that not merely of iron against flint or other very hard body, but even of copper against copper, iron against marble, lead against iron or lead, provided the force of the shock be sufficient to develop the required amount of heat, the chemical affinities between the elements of these solid substances are excited; and they are almost instantaneously transformed into gaseous bodies in a state of great compression, and therefore endowed with an enormous elastic force. This chemical change is also accompanied with a great development of heat, which further tends to expand the gases; so that, at the moment of ignition, if considered instantaneous, the gases produced would be capable of expanding to a bulk 2000 times greater than the original volume of the gunpowder, or at that moment would exercise a force capable of reducing 2000 volumes to one. Such a force, if really exerted at its maximum of intensity, would require an enormous thickness, and therefore weight of metal, to resist its expansive effect; but the ignition of gunpowder, however rapid, is not instantaneous, and the ball, therefore, is put into motion, and the resistance of the air overcome before the perfect development of the gigantic power which is thus called into action.

In gunpowder, viewed theoretically, nitrate of potash, nitre, or saltpetre, is the reservoir of oxygen; charcoal, the element which, by its combination with oxygen, should form the gaseous body carbonic acid; and sulphur, the element which should facilitate ignition, and, by its affinity for the metallic base of potash, potassium, assist in inducing the decomposition of the nitre. The chemical formula which represents a compound which after ignition should yield only sulphuret of potassium, carbonic acid, and nitrogen, is KONO₃ + 3 C + S, or, nearly, nitre 74-75, charcoal 13-84, sulphur 11-87; and though the gaseous product might be increased in volume by introducing sufficient carbon to produce carbonic oxide instead of carbonic acid, the gain in volume would be more than counterbalanced by the loss of heat.

The gunpowder of various nations approximates more or less to this normal composition.

| Country | Nitre | Charcoal | Sulphur | |---------|-------|----------|---------| | England | 75-00 | 15-00 | 10-00 | | France | 75-00 | 12-50 | 12-50 | | Russia | 73-73 | 13-99 | 12-63 | | Austria | 76-00 | 11-50 | 12-50 | | Spain | 76-47 | 10-78 | 12-75 | | Sweden | 75-00 | 9-00 | 16-00 | | China | 75-00 | 14-40 | 9-60 |

It must not, however, be imagined that the result of ignition is exactly that which, from the theoretical formula, should be expected, as the products of combustion are more varied, and include, in addition to sulphuret of potassium, carbonic acid and nitrogen, portions of carbonic oxide, sulphuretted hydrogen, carburetted hydrogen, sulphide of carbon, sulphate and carbonate of potash, cyanide of potassium, and vapour of water; these products being the results of progressive, and therefore partial, ignition and decomposition. According to Dr Ure, the gaseous products of the combustion of gunpowder are carbonic oxide, sulphurous acid, and nitrogen; and there can be no doubt that the comparatively imperfect operation of chemical affinities between the particles of its solid constituents has a tendency to introduce more or less of sulphurous acid into the results by the direct combustion of the sulphur. Some of the modifications in the composition of gunpowder adopted by various countries have most probably been directed to a correction of this evil, as both sulphurous acid and sulphuretted hydrogen exert an injurious action on the metal of guns, more especially on that of bronze guns. As general rules, however, the sulphur must not be much less than the quantity of the theoretic formula, as, if so, the rapidity of ignition, and the amount of heat will be diminished; nor must it be much more, as, if so, the production of sulphurous acid will be increased, and with it the destructive effects on the gun; and in like manner the charcoal must not be much less, as, if so, there will be a defect of carbon for forming either carbonic acid or carbonic oxide; nor much more, as the gunpowder would, in addition to other defects, be thereby made too highly absorbent of moisture. When first made, gunpowder is in the state of fine dust, which is called meal powder, but as it is then liable to cake, and from the close packing together of its grains the rapid transmission of flame would be checked, the powder is granulated under a moderate force of compression, by which the grains acquire a greater density; and this quality being aided by glazing, they retain more effectually their form, and in some measure resist moisture, whilst the passage of flame through an assemblage of such grains is comparatively easy. Nothing but experience and practice can enable the manufacturer to decide the exact amount of compression or degree of glazing, as any material excess must injure the quality of the powder, by diminishing its combustibility.

The knowledge of an explosive force equivalent, according to Professor Graham, to at least 1000 atmospheres, and the power of readily applying it to practical uses, must very soon have led to important changes in the machines of war; and it might therefore have been expected that the exact epoch of its first employment would have been easily discovered. Such, however, is not the case, and there is still considerable uncertainty in the histories of gunpowder and of cannon. Colonel Chesney has most ably followed up the researches of M. Reinand and M. Favé on the Greek fire of the ancients, and done much to establish the correctness of their opinion, that compounds of nitre were used at a very remote epoch, not merely for ignition, but also for explosion and propulsion. Passing over the earlier instances of the mention of "incendiary projectiles," amongst the natives of China and Hindustan, Colonel Chesney, quoting the fol-