furnish others which, when pursued to their practical consequences, will serve to correct and improve both the theory and the art of gunnery.

Mr. Robins apprehends that the force of fired gun-powder consists in the action of a permanently elastic fluid, similar in many respects to common atmospherical air; and this opinion has been very generally received:-but Count R. thinks that, though the permanently elastic fluids, generated in the combustion of gun-powder, assist in producing the effects which result from its explosion, its enormous force, allowing it to be 50,000 times greater than the mean pressure of the atmosphere, cannot be explained without supposing that it arises principally from the elasticity of the aqueous vapour generated from the powder in its combustion.

The brilliant discoveries of modern chemists have taught us, that both the constituent parts of which water is composed, and even water itself, exist in the materials which are combined to make gun-powder; and there is much reason to believe that water is actually formed, as well as disengaged, in its combustion. M. Lavoisier, I know, imagined that the force of fired gun-powder depends in a great measure upon the expansive force of uncombined caloric, supposed to be let loose in great abundance during the combustion or deflagration of the powder: but it is not only dangerous to admit the action of an agent whose existence is not yet clearly demonstrated; but it appears to me that this supposition is quite unnecessary; the elastic force of the heated aqueous vapour, whose existence can hardly be doubted, being quite sufficient to account for all the phænomena. It is well known that the elasticity of aqueous vapour is incomparably more augmented by any given augmentation of temperature than that of any permanently elastic fluid whatever; and those who are acquainted with the amazing force of steam, when heated only to a few degrees above the boiling point, can easily perceive that its clasticity must be almost infinite when greatly condensed and heated to the temperature of red-hot iron; and this heat it must certainly acquire in the explosion of gun-powder. But if the force of fired gun-powder arises principally from the elastic force of heated aqueous vapour, a cannon is nothing more than a steam engine upon a peculiar construction; and upon determining the ratio of the elasticity of this vapour to its density, and to its temperature, a law will be found to obtain very different from that assumed by Mr. Robins in his treatise on gunnery.'

Count R. contests another position of Mr. Robins, which supposes the inflammation and combustion of gun-powder to be so instantaneous, that the whole of the charge of a piece of ordnance is actually inflamed and converted into an elastic vapour before the bullet is sensibly moved from its place;' and he also alleges that the ratio of the elasticity of the generated fluid to its density, or to the space occupied by it as it expands,

is very different from that assumed by Mr. R.:--but for the observations and experiments that relate to these subjects, we must refer to the Transactions at large.

In order to measure the elastic force of fired gun-powder, Count R. adopted a new plan; and, instead of causing the generated elastic fluid to act on a moveable body through a determined space, which he had found to be ineffectual to his purpose, he contrived an apparatus in which this fluid should • be made to act, by a determined surface, against a weight, which by being increased at pleasure should at last be such as would just be able to confine it, and which in that case would just counterbalance and consequently measure the elastic force.' Having succeeded in setting fire to the powder, without any communication with the external air, by causing the heat employed for that purpose to pass through the solid substance of the barrel, it only remained to apply such a weight to an opening made in the barrel, as the whole force of the generated elastic fluid should not be able to lift, or displace.' Many precautions were necessary. The author's apparatus is minutely described and illustrated by a variety of drawings, to which we must refer.

Of the astonishing force of fired gun-powder, some judg ment may be formed from the following experiments, which we shall briefly recite. Having charged the barrel with ten grains of powder, its whole contents being about 28 grains, and a 24 pounder, weighing 8081 lbs. Avoirdupois, being placed on its cascabel so as by its weight to confine the generated elastic fluid, a heated iron ball was applied to the end of the vent tube. In a few moments, the powder took fire, thoughr the explosion made a very feeble report; and when the weight was raised, the confined elastic vapour rushed out of the barrel. The slight effect produced by this explosion induced some of the attendants on this occasion to undervalue the importance of this experiment, and to form a very inadequate idea of the real force of the elastic fluid that had been thus almost insensibly discharged.In a second experiment, the barrel was filled with powder, and the same weight laid on as before. The barrel was made of the best hammered iron, and uncommonly strong. The charge of powder amounted to little more than of a cubic inch, which is not so much as would be required to load a small pocket-pistol, and not onetenth part of the quantity frequently used for the charge of a common musket. Yet this inconsiderable quantity of powder, when set on fire, exploded with a force that burst the barrel, and with a loud report that alarmed the whole neighbourhood.

The author proceeds to make an estimate, from the known strength of iron, and the area of the fracture of the barrel in the preceding experiment, of the real force employed by the elastic vapour to burst it; and he computes that it must have been equal to the pressure of a weight of 412529 lbs. ; which, by another computation, he found to be 55004 times greater than the mean pressure of the atmosphere. By another process, he investigates the strength of the iron of which the barrel was made; and he thence finds that the force required to burst it was equal to the pressure of a weight of 410624lbs, This weight, reduced into atmospheres, gives 54750 atmospheres for the measure of the force exerted by the elastic fluid in the present instance. This force must be considerably less than the initial force of the elastic fluid generated in the combustion of gun-powder, before it has begun to expand; for it is more than probable (says Count R.) that the barrel was in fact burst before the generated elastic fluid had exerted all its force, or that this fluid would have been able to have burst a barrel still stronger than that used in the experiment.'

For other experiments conducted under his direction, and reported by the two gentlemen who were employed in performing them, we must refer to the Count's own particular detail. The results of all of them are arranged in tables; from which he has deduced an equation, expressing the relation which the given densities of the generated elastic fluid, (or, which amounts to the same thing, the quantities of powder used for the charge,) will at all times bear to the different corresponding elasticities of the generated fluid. From all the Count's experiments, it appears that the elasticities, which, according to Mr. Robins, are as the densities, increase faster than in the simple ratio of the corresponding densities.

The author suggests, by way of practical conclusion, that the readiest method of increasing the effects of gun-powder is to accelerate its inflammation and combustion; for he alleges that the slowness of its combustion has prevented the discovery of its enormous and almost incredible force. In order to produce this effect, he proposes to set fire to the charge of powder by shooting, through a small opening, the flame of a smaller charge into the midst of it. He contrived an instrument for firing cannon on this principle, which would supersede the necessity of using priming, or of vent-tubes, portfires, and matches, and thought it might be of use in the British navy but it does not appear to have been received into practice. Another method of increasing the effect of gunpowder would be to cause the bullet to fit the bore exactly, or without windage, in that part of the bore at least in which the

bullet

bullet rests on the charge. The Count has in his possession a musket, from which, with a common charge of powder, he fires two bullets at once with the same velocity that a single bullet is discharged from a musket on the common construction, and with the same quantity of powder; and this advantage he ascribes to the means that are used for effectually preventing the loss of force by windage.

At the close of this paper, we have a computation, designed to shew that the force of the elastic fluid generated in the combustion of gun-powder, enormous as it is, may be satisfactorily explained on the supposition that it depends solely on the elasticity of watery vapour, or steam. From experiments made in France in the year 1790, it appears that the elasticity of steam is doubled by every addition of temperature equal to 30° of Fahrenheit's thermometer. As the heat generated in the combustion of gun-powder cannot be less than that of red-hot iron, it may be supposed equal to 1000° of Fahrenheit's scale:-but the elastic force of steam is just equal to the mean pressure of the atmosphere, when its temperature is equal to that of boiling water, or to 2; 2° of Fahrenheit's thermometer; consequently, 212°+30°-242° will represent the temperature, when its elasticity will be equal to the pressure of two atmospheres; and, pursuing the calculation, at 602°, or 2° above the heat of boiling linseed oil, its elasticity will be equal to the pressure of 8192 atmospheres, or above eight times greater than the utmost force of the fluid generated in the combustion of gun-powder, according to Mr. Robins's computation: but the heat in this case is much greater than that of 602° of Fahrenheit, and therefore the elasticity of the steam generated from the water contained in the powder must be much greater than the pressure of 8192 atmospheres. At 722°, the elasticity will be equal to the pressure of 131,072 atmospheres; and this temperature is less than the heat of iron, which is visibly red-hot in day-light, by 355°:-but the flame of gun-powder has been found to melt brass, which requires a heat equal to that of 3807° of Fahrenheit; 2730° above the heat of red-hot iron, or 3805° higher than the temperature which gives to steam an elasticity equal to the pressure of 131,072 atmospheres. That there is in gun-powder water sufficient for supplying the necessary quantity of steam, the author has very satisfactorily evinced: but we must not pursue his curious investigations any farther.

Farther Experiments and Observations on the Affections and Properties of Light. By Henry Brougham, jun. Esq. From the first series of experiments recited in this paper, and which may be considered as a continuation of those for

merly

merly noticed, (see M. R. vol. xxiii. N. S. p. 42, &c.) the author deduces the following general conclusions: that, when homogeneal light is reflected, some rays are constantly disposed into larger images than others are, that is, into images more distended in length, though of the same breadth; and that the same effect occurs when light is inflected and deflected, and likewise when the rays are refracted in a way analogous to that in which the other images were produced by reflexion and flexion. He then proceeds to shew that this difference of size is not owing to the different reflexibilities and flexibilities of the rays. From a train of reasoning, which we have not leisure for pursuing, he thinks himself warranted in inferring,

that different sorts of rays come within the spheres of flexion, reflexion, and refraction, at different distances, and that the actions of bodies extend farthest when exerted on the most flexible. To this property of light he gives a new name, and accordingly observes that the rays of light differ in degree of refrangibility, reflexity, and flexity, comprehending inflexity and deflexity. These terms allude to the degree of distance to which the rays are subject to the action of bodies.

The author's next object was to measure the different degrees of reflexity, &c. of the different rays: but none of his measurements authorize him to conclude, with any certainty, that the action of bodies on the rays is in proportion to the relative sizes of these rays but this, he thinks, may be found to be the case; and in the mean time there is little doubt that the sizes are the cause of the phænomenon.

Having established his principles, Mr. B. applies them to the explication of several phænomena in vision. He then states, very much in detail, the consequences that follow from the principles which he had laid down in his former paper, and which he has verified by subsequent experiments, to his full satisfaction, and even beyond his original expectations. The chief consequences are the following, viz. that a speculum should produce, by flexion and reflexion, colours in its reflected light wherever it has the least scratch or imperfection on its surface:-That, on great inclinations to the incident rays, all specula, however pure and highly polished, should produce colours by flexion:That they should also, in the same case, produce colours by reflexion :-That lenses, having the smallest imperfections, should produce by flexion colours in their refracted light :-That there should be many more than three, or even four fringes by flexion, invisible to the naked eye; and that Iceland crystal should have some peculiarities with respect to flexion and reflexion; or, if not, that some information should be acquired concerning its singular properties respecting refraction.

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