326 On guiding the Course of Balloons. 



performed. When there is a very strong wind blowing, let » 

 stone, a clod of earth, and a piece of paper, all having the same 

 form and equal surfaces, he dropped together from a smalT 

 height. It mav then be observed, that in the smne t!?nf, the 

 paper will be carried to a considerable distance by the uir.d, the 

 clod of earth to a less distance, while tiie stone is scarcely nioved 

 at all. This certainly tends to establish the fact, that a dimi- 

 nution of velocity does take place, which I am led by analogy to, 

 believe is in the sub-duplicate ratio of the difference between 

 the specific gravity of the body and that of the air Hence, as 

 the plan I have described recjuires the balloon never to be in. 

 equilibrio, it mav reasonably be expected to move with a velo- 

 city considerablv less than the wind. 



I shall just observe, that the plane may with very slight alter- 

 ations be made to serve as a parachute, which may be directed 

 on the same principle as the balloon; and from the experiments 

 I have made with parachutes thus formed, it appears to be 

 greatly superior to that used by Mons. Garnerin, both as to se- 

 curity and uniformity of motion, being entirely fiee from oscilla- 

 tion. 



It may perhaps appear unnecessary to some persons that 1 

 have had recourse to an oblique plane, independent of the bal- 

 loon, instead of constructing the balloon itself of such a form 

 as to present to the air the required oblique surface. I have, 

 lately tried several experiments with balloons of different shapes, 

 and which have convinced me of the great difficulty in attaining 

 the object bv such a method. But it would extend this paper 

 to an excessive length, if I were to detail the various experiments, 

 and explain the causes of their failure. 

 I am, sir. 



Yours respectfully, 



Islington, Oct. 19, 1815. JoHN EvANS, Juil. 



P.S. — The method of forming the gores of a balloon, which 

 is given by Cavallo, and after him in most of the Encyclopjedias, 

 is so very tedious, that any abridgement of this essential process 

 must be considered an advantage. A brief description of the 

 construction which I have employed for some time may, per- 

 haps, prove acceptable, on account of its great simplicity and ac- 

 curacy. It is derived from the obvious property, that the breadth 

 of the gore in any particular part is proportional to the chord 

 drawn throui^h that part parallel to the equatorial diameter. 



By the usual methods find the middle breadth AB (fig. 4) and 

 the length CD of the gore. On AB as a diameter describe the 

 required sha|)e AEBF ot the balloon. Divitle the curve line FAE 

 and the straight line DC into the same number of equal parts, 

 and through the points of division dravi' the straight lines gh, ik. 



