While examining the cask, to find if there was not some means of ascertaining its different diameters, I discovered the very way itself. All I wanted was a straight rod or stick, of sufficient length to reach quite across the butt at its thickest part. It was plain to me, that by inserting such a stick into a hole in one side of the cask, and passing it on till it touched the staves on the other side, at a point diametrically opposite, I could thus obtain the exact measurement of the diameter of that part of the vessel, since the portion of the rod reaching from side to side would be the diameter itself. The diameter once obtained, it needed only to multiply by three to get the circumference. But in the calculation which I was desirous of making, it was the diameter itself I wanted to find, and not the circumference. I only thought of the latter, because, under ordinary circumstances, when a cask is bunged up, it is easier to measure the circumference of the swell than its diameter. In no case does it signify which, as the figure three will always reduce the one to the other, near enough for most practical purposes, though not mathematically exact. Now, it so chanced that one of the holes I had cut through the staves had been made in the very middle of the swell, where the butt was thickest. Therefore a straight stick passed into this hole, and pushed on till it touched the opposite side, would give the greatest diameter of the cask. You may imagine that this might have been obtained by simply planting the stick in a vertical position _outside_ the butt, and notching it at a point on a level with the top of the vessel. True, this might have been done had I been operating with a barrel lying upon a plain surface, with nothing around it to obstruct me, and plenty of light to observe the true level. Even thus it would have been rough guess work, and not to be depended on when a calculation was to be made involving life or death in its consequences--for such it really did involve--at least, I supposed so. But the butt was so placed, resting upon the timbers of the ship, with its swollen side sunk between them, that I could not have measured it in this manner. Even though I might have marked a rod on a line with its top, I could not have planted the other end so as to be on a level with its base. There seemed no other way to get at the thing than by inserting a straight stick into the hole, and thus measure the diameter; nor did I trouble myself about any other, as this appeared to be the best plan I could adopt. Where was I to find my stick--my measuring-rule? That is your question, is it not? It is easily answered. The deal board that had formed part of the biscuit-box would supply me with the material, and out of that I could soon make one. No sooner thought of than I set about it.