same apprentices may be employed that are now engaged, but not in the same dangerous and pernicious way; so that the master in the art will have no interest in opposing the alteration of the practice.

The members of parliament who shall be instrumental in forwarding this wholesome law will receive the blessings of those oppressed little beings whom it will relieve, and the praises of all mankind.

On the Employment of the Aerostatic Machines in the Military Science, and for the Construction of Geographical Plans. By A. F. LOMET, Adjutant-commandant, formerly Keeper of the Models at the Polytechnic School, and now Chief of the Sixth Division of the War Department.

With a Plate.

From the JOURNAL DE L'ECOLE POLYTECHNIQUE.

AEROSTATION is yet in its infancy; it is therefore

of importance to obtain the assistance of learned men and artists in this interesting pursuit; but as investigations in this art are generally too expensive for individuals, it seems necessary that government should support an establishment particularly devoted to the practice and improvement of it.

Aerostats will furnish, in presence of an enemy, one or more points of observation at pleasure, from which the positions he occupies may be reconnoitred, his movements studied, and his manoeuvres judged of in the gross, or appreciated in the most minute, detail. It may be presumed that these machines will become of the most indispensable utility in war, because they supply it with an extraordinary means, hitherto unknown, of making obVOL. IV.-SECOND SERIES.

I

servations,

servations, which may in an instant determine the fate of battles, secure the dispositions for a vigorous defence, or at least point out the moment and the most convenient outlets for a retreat; and more particularly to draw attention to the advantages which an army may derive from balloons, it will be sufficient to remember the happy use made of them at the battle of Fleurus.

The Committee of Public Safety, and afterwards the Executive Directory, thought that the application of aërostats to military enquiries of every description ought to be studied and practised during peace. They were also desirous, that they might be employed in the construction of geographical plans, or at least in ascertaining the intermediate particulars of the territory between the points which had been geometrically determined, Having been charged with the experiments relative to these different applications, I purpose giving an account of the principal results.

The intention, from the first ascents, was to measure the angle formed by the visual rays falling on the eye of the aërial observer, from several determinate points on the earth. The unavoidable motion of the aërostat preventing the use of the graphometer in this operation, a recipiangle was at first substituted, suspended like a mariner's compass, by the assistance of which it was hoped the measure of the angles would be easily taken, and particularly that they would be obtained with immediate relation to a horizontal plane. This attempt not having succeeded, it was necessary in future to make use of a

sextant.

This instrument was every thing that could be desired for celerity, as well as for the facility and precision of the observations, but it has this inconvenience, in the case in hand, that it only shews the angle on a plane inclined to

the

the horizon; and moreover, in its ordinary construction, it furnishes no means of noticing this inclination. The perpetual agitation of the machine is another source of error; in fact, an aërostat, kept elevated and held by cords, is continually changing position; it moves in space, describing alternate ellipses, the curvature of which is modified to infinity, according to the violence of the wind, the elasticity of the cords, and the situation of the places to which it is fastened. It leaves then no trace of its variations, and does not permit the observer, which it supports, to add to the measure of any angle whatsoever, that of the two angles necessary to connect the first with the plane of the horizon.

Nevertheless, for plans relative to the generality of military inquiries, and in all cases where a sketch of the figure of the earth is sufficient without attending to slight inaccuracies in distances, simple observations, made with the sextant, will answer the purpose, and furnish the means of operating with facility over a vast extent of territory, secure from the attempts of an enemy. But it is not equally serviceable in operations which require a rigorous exactness, and in which it is requisite to connect the angles with the centre of the station, and with the plan of the horizon.

The following is the mode in which I have endeavoured to fulfil the various objects:

The angles necessary for connecting the position of two objects with the centre of the station and the plane of the horizon, are, first, the angle comprehended between the rays falling on the eye of the observer from these points ¡ second, the angles formed by each of these rays with the perpendicular. We have seen, that it is impossible for the aërostatic observer to mark these three angles by taking them after each other; but if their measure were instan

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taneous, the difficulty would be overcome. This would. therefore be the case if an instrument could be devised, which would give these three angles at once by a single observation; and as the sextant already shows the angle comprehended between the rays, the object in view is to add the necessary parts to that instrument for obtaining the other two at the same time.

Let BAC (Fig. 2, Pl. III.) be the angle formed by the visual rays A B, A C; falling on the mirror A of the sextant from the objects B and C; if the index A D be moved until the image of the object C reflected by the mirror A placed on the index coincides by double reflection on the mirror L, with the point where the object B is seen; and if they be both perceived at the same time by the observer looking through the telescope P, it is known, (by the theory and use of the sextant,) first, that the angle D A E, comprised between the index A D and the fixed radius or line of zero A E of the instrument, is always equal half the angle BA C, the measure of which is required: second, that the line R B, which is supposed to pass through the axis of the telescope and the centre of the mirror L, is always directed to the point B, and is usually taken for the side A B; the error arising from the small distance AR being considered as nothing in practice: hence, if we suppose a visual ray passing from the point R to the object C, the angles B R C and BAC may be reputed equal, and be taken indiscriminately for

each other.

This being premised, if a ruler be placed in the direction RB, it may be considered as in that of the side A B, and if we can succeed in fixing a second ruler in such a manner that the moveable index shall carry it into the direction R C, at the instant that the images of the two objects B and C are brought into one at the point L,

it is evident that these two rulers will form between them the angle B R C, and consequently the angle B A C.

To accomplish this, let us suppose a sort of false square, SRQ, situate in the plane of the instrument, and moveable at its axis on a pivot fixed at the point R, at the intersection of the lines AR and RL; making the angle SRQ, comprised between its arms, equal to the angle ER L, and the side R equal to the distance A R. If now we suppose that the extremity S of the side RS is retained by a button in a groove MN,. worked in the moveable index, the movement of it will be communicated to the false square in such a manner that the angle LRQ will always be equal to the angle BAC, and consequently the side R Q will be placed in the requisite direction.

In fact, the triangle ARS being isosceles in its form, the exterior angle SRE RAS+ASR. 2 RAS= BAC; but SRQ being equal to ERL, if the common angle SRL be taken away, there will remain the angle SRE LRQ=BAC.

Now let us fix, under each of the two rulers R L and QR a small graduated quadrant, suspended in such a manner that it will place itself in the vertical plane of the. side corresponding to the angle observed; let us affix to each of these quadrants a plummet, composed of a stiff arm, moveable upon a pivot, and furnished with a no-. nius index and a weight, which gives it a constant tentency to assume a vertical position, in whatsoever situation the sextant may be placed; finally, let the whole be so disposed that the index of each plummet may be retained at will, at the division indicated on the limb by the effect of the suspension, and this by means of a trigger, which can be pulled at the exact instant of observing the principal angle in the points of reflection. It is

evident