PRACTICAL REMARKS ON AERIAL NAVI- Sir,-Permit me, through the pages of the Mechanics' Magazine, which widely clrculates among the efficient mechanics of this engineering age, to call their attention to a subject of great national interest, and one that offers perhaps the most difficult triumph of mechanical skill over the elements man has to deal with-I mean the application of aerial navigation to the purpose of voluntary conveyance. There seems to be, if we may judge by the scattered notices in the public journals, a revived attention to this subject, not only in this country, but also in France and America: the experiments that have been made, and the investigation which it has undergone, lie almost unconnected in the periodical publications of the last thirty years; and hence every new speculator on the possibility of steering balloons, takes up the subject merely on his own view; and as it requires much complicated calculation, as well as the utmost exertion of engineering skill, it is not surprising that we do not make much progress, especially when we consider the enormous expense of making experiments upon it on an efficient scale of magnitude. Among others, five-and-twenty years ago, I paid considerable attention to the subject of aerial navigation, and collected or ascertained several of the leading points and laws of action that must be complied with to render any attempt respecting it successful. These were published in the Philosophical Magazine for 1816 and 1817, &c. I shall not, however, repeat much of what is there said, but proceed to state what I consider most conducive at present towards a final accomplishment of the aerial object in view. In the first place, the enormous bulk of balloons, as compared with the weight they will sustain, causes the difficulty of impelling them, with sufficient speed to be of any utility, either by manual or engine power; and this difficulty is by many This letter was commenced about four months ago; I am extremely glad to see how much this subject has grown upon public attention since that time, and how many excellent notices of it are contained in the last three monthly numbers of truly scientific persons considered as insurmountable, because they conceive that the bulk, which causes the resistance, must ever be commensurate with the weight of engine necessary to propel them by any species of waftage-and, consequently, as it will not do on a small scale, that it can not on a large one. It is true, that it requires twice as much gas to sustain a 4-horse power engine as to sustain one of a 2-horse power (with their loads of fuel and water); but it is not true that the larger balloon, though perfectly similar in make to the smaller one, will, when driven through the air at the same velocity, meet with double the resistance— if it were so, the case of steering balloons would be hopeless, and on this mistaken ground many think it a vain attempt. This idea, resting at the very threshold of the invention, and which seems to present an insurmountable barrier, when probed and fully investigated proves to be false, and the investigation leads to an immutable law of proportion between the resistance and the capacity to carry weight or engine-power, which, on a very large scale, promises the most satisfactory result. If balloons of the respective diameters of one and two, both being spherical, be driven through the air with equal speed, the resistance will be as the surfaces opposed to the air, and the surface of the largest will be four times greater than that of the smaller, and hence it will re quire four times the engine force to keep up the velocity; but the quantity of gas contained in the larger balloon is eight times greater than that in the smaller, hence it could sustain eight times as much engine-power; but four times that power would keep up the required velocity, and hence it could carry a cargo of the weight of its engine, and yet keep pace with the smaller balloon. The simple terms of the case are, that the surfaces (and hence the resistances) increase loon; whereas the capacity to contain as the squares of the diameter of the balgas (and hence the supporting power) increases as the cubes of the diameter. From this unquestionable law it follows, that if similar shaped balloons vary in diameter as the numerals, 1, 2, 3, 4, 5, &c., the resistance they will meet with in the air, at the same velocity, when com your Magazine up to September, which I have just pared to the weight (or engine-power) they will sustain, will be as 1, 1, 1, 4, received. t, &c. This is a most important fact, and proves that as the law of relative di minution to resistance is unlimited, there must ever be, theoretically, some bulk in which any species of first mover, however sluggish in proportion to its weight, would find itself suspended, and its power adequate to propel that bulk with the velocity required. So far for the principles in action; let us now come to the real practical limits and bearing of the case. The The first thing that presents itself to our notice is the choice of a proper material of which to form a balloon for the required purpose; and the properties are those of being perfectly air-tight, light, and strong. Silk and Indian-rubber varnish are thus indicated, and have long been used; but in the larger constructions, that are suggested by the previous investigation, the expense of silk would almost prove a bar to real use. double-cotton Indian-rubber cloth, used by Mr. Macintosh in his manufacture of air-tight seats and cushions of various kinds, weighs very nearly 1lb. per square yard, and will just sustain a tension of 2,500lbs. per lineal yard, that is, if the yard of cloth were rolled up and used like a rope, it would sustain any weight less than 2,500lbs. Of course, if used flat, as a portion of the surface of a balloon, it would sustain tension to the same amount. This cloth, when made to adhere to an adjoining breadth by an overlap of one inch with the Indian-rubber varnish, is air-tight at the seam; and is to the full as strong in resisting tension as at any other part, as I have found by experiments carefully made for the purpose. As we now travel by railroad pretty constantly at the rate of 20 miles per hour, aerial navigation, though offering a direct navigable ocean to every point of our globe, would scarcely be worth cultivating, if not practicable ultimately at least up to that speed. To be able to sustain the form of the balloon, when driven against the air with that velocity, implies that the condensation within must press rather more than the resistance of the external air; but at that velocity, by the well-known laws of resistance, every square yard near the centre, facing the line of flight, will meet a resistance of about 291bs.; and hence the condensation over the whole interior of the balloon must give 29lbs. pressure per square yard. More than this, balloons, to be really serviceable, must when at anchor, or by accident driven against obstacles, be able to resist the action of our most violent storms, which, according to Smeaton's table, go at the rate of 60 miles per hour, tearing up trees, and creating a pressure of 162lbs. per square yard.* This cloth can just sustain 2,500lbs. per lineal yard; and hence, by calculating the forces, it follows that the extreme limit of size to which a spherical balloon made of it could safely be carried when occasionally condensed to meet our storms, whilst at anchor, or when compressed against objects casually by them, would be 60 yards in diameter.t Let us not be startled at this deduction, for in practice we may use as much less as we find convenient; and it is a feature of very great importance in favour of aerial navigation, that such a slight fabric is capable of becoming a safe vehicle of support to so vast an extentand here it should be remarked, when balloons are made of forms differing from the sphere, that where for any considerable length they approach, as in elongated spheroids, to a cylindrical form, the cloth will only sustain near the minor axis half the pressure or condensation it will sustain as when in a sphere of the same *These calculations are based upon the resist ance being in the ratio of the squares of the velo cities; and that a velocity of 21 feet per second gives a resistance in air of 1lb. to the square foot. †The whole lineal measure of the circumference (being, in round numbers, 188 yards, capable of bearing 2,500lbs. each), can bear 470,000lbs.; whereas the whole area of the great circle on which the pressure takes place, being 2,840 square yards with a pressure of 162lbs. on each, only amounts to 460,0 '0lbs. In speaking of condensing the gas in balloons, which is a new feature in them, it will be necessary to provide a safety outlet by bringing a wide pipe from them, and placing the end of it a few inches under water; a column of 3 inches of water would equal the condensation of 162lbs. per square yard. The escape of gas should be into a small empty balloon above the water, from which it can be pumped out at pleasure; the change of temperature in the climate also requires this structure. Eventually balloons will probably have a double casing, with common air, or, what would be safer, azote, pumped in between them. A small balloon to contain common air pumped into it, having a tube from it with its mouth a certain number of inches under water, and its bulk contained within the gas-balloon, would be the readiest way to meet all cases of condensation and expansion. With air-tight materials there would be no mixture of common air with the hydrogen, but this plan would require the materials to be perfectly so. To prevent danger from the fire of the engine, several wire-gauze divisions should be made in the chimney. diameter; hence 30 yards would be the extreme limit of the shorter axis of an elongated spheroidal balloon made of this cloth. As the netting, belting, or whatever means be adopted to enable the floatingpower of balloons to sustain the burthens attached to them, must necessarily extend over more than half their surface, it would be best to complete the circuit, and thus add the strength of the netting to resist condensation, and fortify the cloth, especially near the shorter axis. Condensation is a term that seems, and to a certain extent is, adverse to aerial navigation; but the whole condensation here required will only deduct 1lb. of buoyancy from every 120lbs. previously exerted by the gas, a sum too trifling to be of any consequence, and abundantly redeemed by the firmness it gives to whatever form it may be required to model balloons for obviating as much as possible the resistance of the air.* The next consideration is the proper form of the balloon for this purpose; and here it is obvious, that to extend their length horizontally, and thus to diminish their cross-section, is the leading point of the investigation. This will be limited by the practicable extent to which the structure can be carried without incurring weakness, in respect to the preservation of form, or inconvenience in the mode of suspending the car or body from the balloon. Ships and boats range between three and six times the measure of their greatest cross-section; birds between two and four. When convenience has pointed out the limits that must guide us in making use of length to obviate resistance, the form of the balloon, to meet the least resistance within these limits, is naturally the next inquiry. Unfortunately even the sagacity of Newton has not been able practically to grapple with this very interesting and intricate question; and his beautiful theorem on this subject will not apply to any of our gross fluids, which wedge themselves up by accumulation after they have struck upon the resisting body, and have no free egress to make room for others. The New The expense of using pure bydrogen gas points out the necessity of balloons being perfectly airtight, and when used as permanent vehicles, and on the true scale of magnitude, they will probably be made of thin metallic sheets kept firm by condensation, with separate light bags of gas within. p. tonian solid of least resistance has a prow concave near the anterior axis; and as air is more elastic than water, the prow, if we may use the term, of birds is also concave; whereas in fishes the prow, as in ships, is convex. In the absence of all good authority, I have proposed (at 400 of the Philosophical Journal for 1816) to copy the prow of the woodcock as there given by exact measurement. This bird was selected from its having frequently to pass 500 miles of sea at one flight; and because in its structure Na ture seems to have united every contrivance to blend strength with lightness. The resistance of the air to its passage was the great obstacle to be overcome; and hence it is more than probable the best form (which, more than all the rest, would tend to the ease of the performance) has been selected also. It is about 3 times the length of its greatest cross-, section. The hinder portion of resisting solids is proved by experiment to be of as much importance as the prow; but as its office is to fill up the space, shielded from pressure for a time by the diverging momentum of the fluid driven off by the prow, any figure approaching to that of the cone answers the purpose tolerably well, if we may judge by the lengthened conical taper of the tails of fish; indeed it is a common expression among sailors, that a ship to sail well should have a "cod's head and a mackerel's tail." We may rest contented to make our experimental balloon of an extended spheroidal form, and leave the rest to future improvement. The best form of ships remains in a great measure to be ascertained yet; although navigation has been bestowing wealth and comfort on mankind for so many ages under a rude approximation to it. The next objects of inquiry are the power to be used in propelling the balloon, and the means of applying that power. Only two general modes have, I believe, ever been proposed by competent persons for this purpose. My friend Mr. Evans (see the Philosophical Magazine, for 1815, vol. xlvi., p. 321,) tried with success to steer a small Montgolfier balloon by suspending a large oblique surface beneath it, which caused the ascent to be oblique in the direction towards which the upper edge of the plain was pointed; when the fuel failed, gravita tion made its return obliquely to the place from which it set out; had this plain been reversed when at the top of its rise, steerage towards the same point of the compass would have been effected in both cases by this sort of vertical tacking. This movement implies the use of the fire-balloon; but it does not follow that the whole support must necessarily be given on that principle: suppose that twothirds of the weight of the whole machine were suspended by a hydrogen-gas balloon, by any sufficient length of cordage (say it required from 50 to 100 yards), to ensure all danger from fire. Immediately above the car place a fire-balloon, likewise capable, when fully inflated, of supporting two-thirds the weight of the whole apparatus. When both balloons operated upon a large oblique inclined plain, with a power of ascension equal to one-third of the whole weight, it would render the oblique force very efficient in ascending; and when at the highest point of elevation the heated air is let off by the valve, and the plain reversed, one-third of the whole gravitation would give it an equally effective oblique descent. A machine on this construction would, on account of its progressive motion, obey a rudder, by which more exact steerage could be effected. It is certainly the most simple and least expensive way of primarily effecting the problem of steering balloons; but there is something unsatisfactory in being obliged thus to resort to such alternating heights and descents, implying such sudden changes of temperature, to say nothing of the devious and prolonged nature of the track, and the consequent waste of power. I send you a rough and hasty sketch of such a combination of balloons, having a large inclined plain suspended between them, capable of being pointed obliquely for either tack by cords from the car, as shown by the present position of the plain, and the dotted line A B, plate 1. The balloons are made in such proportion to each other as to be of equal power, their contents being as ten to four* very nearly. Several strong ropes should pass from the collected cordage of the upper balloon through the interior of the lower one, as exhibited by the dotted lines, and be made fast to a large hoop forming the top 27 ounces per square yard in hydrogen-gas; 11 ditto hot-air according to the French experiments on Montgolfier balloons. of the chimney; this is partly held up by the two light masts C and D, and forms the means of suspending the car from both balloons, as the cordage from the netting of the lower balloon is also collected on this hoop. From the hinder mast C a sail may be conveniently braced to either side, so as to act as a rudder, and thus preserve a steady course. It is necessary to have a long chimney in Montgolfier balloons, when speed is required, to give sufficient pressure within to balance the external resistance, this must be 75 feet long to balance the resistance of a balloon at 20 miles per hour; and is, no doubt, a great inconvenience in this mode of action. The balloon sketched is supposed to be 30 feet in diameter, with 10 feet of chimney, thus giving 40 feet of columnar height at the top of the balloon, which would create a pressure equal to the external resistance, at a velocity of about 14 miles per hour. I do not offer this as a finished model, but merely sufficient to exhibit the principles in a visible form. Steam, or any mixture of it with heated air, does not offer much prospect of advantage in filling Montgolfier balloons, and if it did, it could only be used when the balloons were of enormous magnitude, so that the proportional diminution of surface and increase of thickness in the materials, had greatly diminished the condensing power of the external air with Indian-rubber cloth of 1lb. to the square yard, which would nearly absorb all the power of the 30 feet balloon I have specified, it would (according to some experiments I made, on cooling, with that cloth,) take about 100lbs. of coke per hour to supply its condensation of steam at the ordinary temperature of our atmosphere. Let us now consider the more direct plan of steerage by the waftage of surfaces to which engine power is applied. Some persons doubt whether if such power can be conveniently suspended to balloons, it would be efficient, because there are not two fluids to work upon, as the water and air in the case of ships; but this argument does not apply since the introduction of the steamboat, where the wind has no concern in the movement; and, indeed we might as well doubt whether the muscular power in the bird's wing is that which propels it forward, as doubt that engine-power, if properly applied to balloons, will have a similar effect. My friend Mr. Goldsworthy Gurney has just completed some steam-carriages, the boiler and engine parts of which weigh no more than 200 weight per horsepower;* the supply of coke and water will be about (10 coke, 60 water) 70 lbs. per hour-say 30lbs. more for the constant quantity left in the fire-place and boiler, and we have each steam horse, with its load for an hour in the weight of 300 lbs. If we take loads for several hours, and use no means of saving water by condensation, which might readily be done, the loads per horse-power will stand thus:: For one hour two four... Lbs. 300 ..... 370 ... 440 This is at present our best practical result. In theory, however, it seems possible that we may obtain a horse-power at very high temperatures, and working by expansion only, at 151⁄2 lbs. of water and 10 lbs. of coke per hour. Lighter first movers than steam-engines may be discovered, and made applicable to propelling balloons; but let us take the case as our experience now places it. Here, in fact, commence the real difficulties we have to contend with in ren.. dering balloons serviceable to mankind; we have as yet only obtained the grounds of calculation, and thsee, when correctly followed up, place the result of the question not on any point of defective theory; but whether it is, or is not, practicable to construct them so as to be firm, air-tight, and manageable, when of di mensions far exceeding any experiments that have been hitherto tried, we must be contented to give up balloons for purposes of locomotion altogether, or to attempt them on that scale of magnitude which a well-grounded calculation of their powers proves to be necessary. It cannot, however, be thought useless, boldly and unflinchingly to investigate the case, which, if practicable, offers us the floatage of an uninterrupted ocean from every man's door to any other point on the globe;-let not such a boon to our race be given up without a fair and vigorous effort to avail ourselves of it in our own age. To commence :-suppose a balloon be made of the Indian-rubber cloth of Messrs. Macintosh and Co., in the form of an elongated spheroid thirty yards in diameter, and three and a half times that measure in length: although this would in bulk bear a strong resem blance to a hundred-gun ship, yet it would fold up into a cubical case 10 feet every way; and when inflated, is only a hollow bag received into a boundless ocean, where bulk ceases to be an inconvenience, calculation proves that a condensation of one part in 120, will give it firmness sufficient to resist storms without affecting its form; and the cloth is known to be air-tight under much more intense condensation: surely, then, we can scarcely doubt the possibility of making such a balloon, or of inflating it by pumping, with pure hydrogen-gas, setting aside at present all consideration of the cost of the experiment. The weight of the materials may be estimated as follows: Indian-rubber cloth at 1lb. to the square yard .... The car .......... The grapples and other matters .... Lbs. 8540 3360 3000 3425 18325 Each cubic yard of hydrogen-gas gives 1.7 lb. of floatage; hence the *Mr. Avery's American rotatory-engine will probably be still lighter if the loss of power do not balance its simplicity of structure. |