CONSTRUCTION OF BALLOONS. 81 



be described as such. But more accurately, the breadths of the slip, 

 at the several distances from the point, to the middle, where it is 

 broade-t, are directly as the sines of those distances, radius being 

 the half length of the slip. After providing the necessary quantity 

 of the stuff, and each piece having been properly prepared with the 

 drying oil, let the corresponding edges be sewed together in such a 

 manner as to leave about half or three quarters of an inch of one 

 piece beyond the edge of the other, in order that this may, in a sub- 

 sequent row of stitches, be turned over the latter, and both again 

 sewed down together, by so doing, a considerable degree of strength 

 is given to the whole bag at the seams, and the hazard of the gass 

 escaping doubly prevented. Having gone in this manner through 

 all the seams, the following method of Mr. Blanchard is admirably 

 calculated to render them )et more perfectly air tight. The seam 

 being doubly stitched as above, lay beneath it a piece of brown 

 paper, and also another piece over it on the outside ; upon this lat. 

 ter pass several times a common fire-iron heated just sufficiently to 

 soften the drying oil in the seam ; this done, every interstice will be 

 now closed, and the seam rendered completely air-tight. The neck 

 of the balloon being left a foot in diameter and three in length, and 

 all the seams finished, the bag will be ready to receive the varnish, a 

 single coating of which on the outside is found preferable to the 

 former method of giving an internal as well as external coat. 



The compositions for varnishing balloons have been variously 

 modified ; but, upon the whole, the most approved appears to be 

 the bird-lime varnish of M. Faujas St. Fond, prepared after Mr. 

 Cavallo's method, as follows : " In order to render linseed oil dry. 

 ing, boil it, with two ounces of sugar of lead and three ounces of 

 litharge for every pint of oil, till they are dissolved, which may be in 

 half an hour. Then put a pound of bird-lime and half a pint of the 

 drying oil into an iron or copper vessel whose capacity should equal 

 about a gallon, and let it boil very gently over a slow charcoal hre 

 till the bird lime ceases to crackle, which will be in about half or 

 three quarters of an hour : then pour upon it two pints and a half 

 more of the drying oil, and let it boil about an hour longer, stirring 

 it frequently with an iron or wooden spatula. As the varnish whilst 

 boiling, and especially when nearly done, swells very much, care 

 should be taken to remove, in those cases, the pot from the tire, and 

 replace it when the varnish subsides ; otherwise it will boil over. 

 Whilst the stuff is boiling the operator should occasionally examine 



VOL, TI. c 



