The strength of powder might be very conveniently and correctly ascertained, by firing a bullet into some uniform medium, such as sand or clay. The medium may first be tried, by letting a ball fall from a given height into it, and observing the depth to which it has penetrated. Since these depths are the spaces through which the ball has passed to lose motion, which is supposed to have been uniformly retarded, they will therefore be as the of their velocities. Hence, when t'..; space has been de termined by the faliwg of which the velocity was known, the first velocity of the projected body may be determined, the space through which it has penetrated being known.
Since the force of gunpowder is exerted in a manner similar to the action of condensed air, which is analogous to the action of a spring, there appears nothing more ne cessary to be known than the volume of elastic fluid set free, and its temperature, to know what is its velocity of expansion when no matter has to be moved.
Mr Robins first ascertained the volume of air formed by a given quantity of gunpowder,*and then supposed that this elastic fluid was exposed to a heat equal to redness, by the explosion of gunpowder. Ile also found, by expe riment, how much the volume of a given bulk of air was increased by a red heat. The first production of air gave him a force of about 244 atmospheres, or that the original volume of gunpowder was multiplied by that number. The red heat he found would multiply this volume by about 4.1. This gave about 1000 times for the increase of volume. The force of its first action would therefore equal so many atmospheres, or 1000x 15 pounds upon a square inch. When this force begins to move, it decreases with the di latation, and exactly in the same ratio. Mr Robins not only ascertained the velocity given to the bullet by a given weight of powder, but he fired the powder alone, which having no weight to move, would expand itself with the greatest velocity. He found that, under these circum
stances, the velocity of expansion was 7000 feet in one second.
In these experiments, the charge, at a small distance, was fired at a piece of wood, constituting the bob of a pendulum. Plate CCLXXXIV. Fig. 12. This pendulum had a ribbon attached to it, which was slightly held between two outfaces, so that, when the pendulum vibrated, the length of the ribbon drawn out measured the cord of the arch of vibration, by which the velocity of the striking body was known. Sec the Description of Plates.
Count Rumford has advanced some new notions respect ing gunpowder, deeming the reasons for its power given by Mr Robins insufficient, viz. the gases and the heat. lie attributes the power of the powder to the vapour of the water it contains, and supposes the heat to be the result of friction. His views of the subject have not been counte nanced by other philosophers, as they appear to he contra ry to experience. Mr Robins, and more recently Mr Cole man, found that powder was the strongest in its most dry state, if none of the materials were injured, and that it be came weakened by the slightest quantity of moisture.
Notwithstanding all the caution used in purifying the ni tre, all powder has a great tendency to absorb moisture from the atmosphere. This may go on to a considerable extent, without any permanent injury to the powder, its power be ing restored by drying. If, however, the water absorbed be sufficient to effect a solution of nitre, the latter after wards crystallizes, and appears like hoar-frost upon the grains of the powder. This is called the starting of the saltpetre, when the powder is not capable of being restor ed to its original strength by drying; for the nitre is, to a certain extent, detached from the other two ingredients. Powder is sometimes so much injured by dampness as to be of no value, except in so far as the nitre can be recover ed from it by solution. (c. s.)