A

Fig. 106.

articulate the propellers to the boat as near to its top as possible, and to place the greater part of the weight of its burden as near as possible to its bottom. This is illustrated by fig. 106, which represents the cross-section of a two-bodied air-craft with parallel suspension, of the species sketched in fig. 89. AC is the horizontal diameter of the gas-vessel; B its centre of buoyancy; DE, the points of suspension on the gasvessel; FG, those on the boat; P, the point of application of the propelling force; w, the place of the centre of gravity of the boat. With such an

arrangement as this, the necessity for compensation by shifting the centre of gravity may be reduced to a minimum. And, of course, the same method applies to the three-bodied craft in which the power-boat is similarly slung. It must, however, be observed that though the axis of balance of the boat is fixed, the centre of buoyancy of the gas-vessel is not, and will be subjected to changes of position with the expansion of the gas. The greater the bulk of the gas, the lower of course its centre of gravity that is, the centre of buoyancy of the float-will be brought, but not in direct proportion to the increase of bulk. This change will necessitate slight adjustments of the position of the centre of gravity of the burden, as the height in the air at which the system is moving varies.

We have now, I think, sufficiently provided, in putting together the vessels of our craft, that the propelling force, when it comes to be applied, shall not derange the level set of the system.

Before quitting the subject of suspension, I must take leave to add a few words in this place on a point which, though not a branch of aërial navigation, is a particular case of the propulsion of gas-vessels. I have already had occasion to mention the possibility of using the buoyancy of light gas as a means of assisting locomotion upon land. It may be interesting to enquire briefly whether it may be possible to render the principle of the neutralisation of weight by floatage serviceable, in assisting us to obtain speed in the other sorts of locomotion on the solid or the

1 See p. 32, above.

liquid surface of the earth. The subject comes properly under consideration here, for it involves all the conditions of aerial travelling of which I have yet attempted the solution, and none of those which are to follow. It must depend, too, particularly for success upon the mode of suspension which might be adopted, in attempting to carry it into practice.

Attempts have been made already to ally the powers of the air to the art of travelling upon land. Matter for amusement on this head may be found in Bishop Wilkins's chapter, ' Of a Sailing Chariot, that may without Horses be driven on the Land by the Wind; '1 wherein he not only gives accounts of instances in which carriages have been propelled by the traction of sails urged by the wind, like the ice-boats of Canada at the present day, but suggests that the effect might be more conveniently obtained by making the wind turn a set of vane-sails which should drive the wheels. The curious reader, too, will meet with entertainment from the kites and char-volant of quaint, pedantic George Pocock, sometime of Bristol, schoolmaster.2 These fancies, however, are not, though aerial, connected with the use of buoyant gas. But balloon railways of different kinds have been proposed. One inventor gravely undertakes to support on balloons lines of railway laid through the air from Dover to Calais, on which locomotive engines and trains shall travel as on the iron network of England. A model of such a scheme was to be seen in the Exhibition in Hyde Park. Others propose to relieve the carriages, which are to run on fixed lines, of their weight by balloons, so that they may be more easily propelled.3 This notion, impracticable to the very last degree, is, however, connected in principle with the point which I am about to discuss. In the same manner it has been proposed to marry the balloon to the sea-boat, so that the keel of the latter just resting on the water shall glide over its surface, taking its

·

' Wilkins, Math. Mag.' Dædalus, c. 2.

2 The Aeropleustic Art, or Navigation in the Air by the Use of Kites or Buoyant Sails,' 4to. 1827. The title is a misnomer-there is not a word in the book on navigation in the air. Its subject is the traction of wheel carriages on land by kites.

'Mech. Mag.' vol. xxvii. p. 249; vol. 1. p. 142; and vol. li. p. 142; 'Aerost. Mag.' p. 79; M'Sweeny, 'Aer. Nav.' p. 71.

propelling purchase from the dense liquid.1 Again, it has been proposed to make buoyant gas do the work of masts in lifting, not the ship itself, but its sails, so as to keep them open to the wind.2 The latter expedient would require the aid of the wind: to the other notions of making gas-floats auxiliary to locomotion on land or water, the wind would be fatal.

I shall not consider whether it would be worth while, under any circumstances, to use buoyant gas-vessels systematically for diminishing the burden of heavy bodies, intended to be transported rapidly over great distances across water or land. The question in all these cases, supposing the air to be still, is whether the resistance of the air to the gas-vessel will be less than the resistance of the water to the part of the boat immersed in it, or than that due to the friction of the wheels of the carriages, and to the necessary lifting of the moving mass over unevennesses in the road. If the sea were smooth, there would be no necessity for such a floated boat being immersed in it at all; its propellers only need dip into the water. But in any case there would still be the difficulty arising from the action of the wind on the gasvessel, a disturbing force which would be fully as injurious to the progress of the system if it was impressed sideways, as if it met the vehicle with direct opposition. We may be able to find our currents in the air aloft, but certainly cannot ensure a favourable breeze below. I shall therefore consider only the question of the possibility of accelerated transit, on a plan which might serve as an occasional expedient or as an amusement, when the wind might be propitious. And I shall take it in the simplest form of all possible human locomotion, that of the man upon his legs, leaving it to the reader if he should feel inclined to wing with gas-float horse, cab, velocipede, or funny, and to make experiment of the result.

If the weight of a man is neutralised by any contrivance, much less force is required to move him than if the burden of his body has to be lifted continually by the moving power. Two appliances are in common use for this purpose in locomotion

upon solid surfaces, wheels and skates. The force applied to the movement of a person supported on either of these forms of instrument would be, of course, if the road or the ice were quite smooth and hard, as completely relieved of the resistance due to his weight as if he were suspended and counterpoised on a beam balanced on a point-so long as the line of action of the force is strictly confined to a horizontal plane. But neither macadamised granite, iron rails, or ice, ever present either of these qualities in perfection the weight of the body has to be lifted continually over obstacles, or to bend the surface that bears it into a depression at the point of contact. Besides which there is in both cases, as well as with the balance beam, considerable friction to be overcome. Again, not the slightest rise out of the plane of level can take place, even if the surface of support be inclined at the exact angle of motion, without a part of the weight having to be lifted by the propelling force, which, if any attempt be made to leave the plane of support, must bear the whole load of the system. If, however, the weight of the body be counterpoised by a buoyant float, the conditions are quite changed. The system is free to move in every direction without any friction, and without any of the weight being thrown upon the moving force. The only impediments to rapid motion are the inertia of the whole system, including, of course, that of the gas-vessel and its contents, and the resistance of the air to the large body immersed in it. The inertia is but a transient resistance, and repays the whole of the force expended in overcoming it, by sustaining the motion afterwards. The resistance of the air, then, is the only opponent to be combated.

The question, then, as applied to the case of a man upon his legs is this-Is the force required to propel through the air at a given speed a gas-vessel just capable of sustaining the weight of a man, less than the force required for him to progress at the same pace without it? Or,-Given the force put forth by a man in locomotion on the ground in the usual manner, can he attain a greater speed by the same exertion, if his weight be counterpoised by a buoyant float, notwithstanding the resistance of the air to the latter? The latter form of the question I shall take as the most convenient.

I have shown before1 that in walking a yard, a man of the average height raises the weight of his body (150 pounds) to a height of three inches. And this is equivalent to raising 12.5 pounds through a yard.2 For every yard, then, that he walks he overcomes a constant resistance, equal to 12.5 pounds, and independent of the velocity of his motion. In walking, then, at any pace which he can continue for a considerable time without distress, a man may be considered as exerting a uniform force of this amount, and just balanced by the resistance to his motion. The rate of four miles per hour is a convenient speed for a pedestrian; at this pace, then, he may be considered as exerting a force equal to a pressure of 12.5 pounds. This, then, is the measure of the force of his legs in easy work.

His weight now is to be counterpoised. According to the table which I give below,3 2,192 cubic feet of hydrogen are requisite to sustain 150 lbs. The dimensions of a gas-vessel of our provisional form and with such contents must be 8.87 feet in diameter, by 53.22 feet in length. The extent of surface of the envelope will be about 1,140 square feet; and if made of varnished silk, weighing 05 lbs. per square foot, will (see Appendix B) weigh 57 pounds; to this must be added the weight of the framework for stiffening the gas-vessel, and that of the suspending cords; this may add another 100 lbs. to the burden. Now, to get this weight supported, we will suppose the greatest diameter of the vessel to be 9 feet exactly, and the length of it to be increased by inserting a cylinder in its middle (diminishing thereby the resistance it will suffer from the air) of sufficient length to counterpoise both the excess of weight already reckoned, and the load of the additional envelope. The length of the piece thus let in may be taken at about 106 feet. This will

The area of a circle 9 feet in diameter is 63.61 square, and its circumference 28.27 linear feet. So that the cubic contents of every foot in length of the required cylinder are 63.61 cubic feet, and its surface 27.28 square feet. Now a cubic foot of hydrogen should lift 0684 lbs. avoird. (see Appendix F. Each foot in length then of our interpolated cylinder will sustain a weight of 63.61 x '0684-4.35 lbs. And its surface of oiled silk