Let us try it, then." To say, "Let us try it," meant the same as to say, "Let us make the balloon." The others understood that; and of course acquiesced in the determination.
As there was nothing to interfere with the immediate commencement of the work, they resolved to set about it at once; and in fact did set about it without farther delay.
The number of eel-skins on hand, though very considerable, would not be near enough for covering a balloon; and therefore Ossaroo went to work with his hooks and lines to catch a few hundreds more.

Karl was able to tell how many it would take; or he could at least make an estimate sufficiently exact for the purpose.

He designed a balloon of twelve feet diameter: for he knew that one of less size would not have power enough to carry up the weight of a man.

Of course, Karl knew how to calculate the surface of a sphere whose diameter should be twelve feet.
He had only to multiply the diameter on the circumference; or the square of the diameter on the fixed number 3.1416; or find the convex surface of the circumscribing cylinder; or else find four times the area of a great circle of the said sphere.

Any one of these methods would give him the correct result.
On making the calculation, he found that a sphere of 12 feet diameter would have a surface of 452 square feet, within a trifling fraction.
Therefore 452 square feet of eel-skins would be required to cover it.
In other words, that quantity would be required to make the balloon.
As the eels happened to be of large size--most of them being over a yard in length, and full four inches in average girth--the skin of one when spread out would yield about a square foot of surface.