For _shot_, there are two straps crossing at right angles, one passing through a slit in the middle of the other. For _shells_, there are four straps soldered to a ring of tin, or fastened to it by cutting four slits in the ring, into which the upper ends of the strap are hooked, and turned down on the inside of the ring. The sabots for 32 and 24-pdr. field howitzers having no groove, each strap is fastened by one nail on the side, and two under the bottom of the sabot. 22. What is a _canister for field-service_? It consists of a tin cylinder attached to a sabot and filled with cast-iron shot. 23. How is it made? The cylinder is fastened to the sabot by six or eight nails, and a plate of rolled iron is placed at the bottom on the sabot. It is closed with a sheet-iron cover after being filled, the top of the cylinder being cut into strips ½ an inch long, and turned down over the cover. 24. In case of heavy guns are the shot attached to the sabot? They are generally without a sabot. 25. How is it with shells? They are strapped to sabots made of thick plank, with strips of tin, as in case of strapping shot for field-service. 26. How is it with canister for siege and sea-coast guns? They have no sabot; the tin is turned over the iron bottom. [Sidenote: 95]] 27. How is it with canisters for the 8-in. siege and sea-coast howitzers? They are attached to sabots in the same way as the field-howitzer canisters. The sabot for the siege howitzer has a hemispherical bottom and the sea-coast a conical one, to suit the connecting surface between the cylinder of the bore and the chamber in these pieces. 28. Are sabots used with grape shot? Yes, in the 8-inch sea-coast howitzer. 29. What is its form, and how fastened? It is conical; and may be fastened to the lower plate with screws, or the pin may be made long enough to pass through it; or else the sabot maybe inserted into the piece separately from the stand of grape. 30. What is the object of fixing shot or shells to wooden bottoms? To prevent injury to brass cannon; and to insure the fuze of a shell being retained in the axis of the piece. 31. What proportion does the weight of one shot bear to that of another? The proportion is, as the cubes of their diameters. 32. How is the weight of a cast-iron shot or shell determined? Multiply the cube of the diameter of the shot in inches, or the difference of the cubes of the exterior and interior diameters of the shell by 0.134 for the weight in pounds. In case of lead balls, the multiplier is 0.214. 33. How is the diameter of a cast-iron shot of a given weight found? Divide the weight in pounds by 0.134, and extract the cube root of the quotient, which will be the diameter in inches. [Sidenote: 96]] 34. How is the quantity of powder which a shell will contain found? Multiply the cube of the interior diameter of the shell in inches, by 0.01744 for the weight of powder in pounds.