Why did people shoot cannon balls into the water where someone drowned, so they would rise to the surface?

Question

This is for a school assignment so please include sources or know what you're talking about. No jokes please!

Answer 1

Radford's Encyclopaedia of Superstition, by E. and M.A. Radford (1947), describes a "widespread" British superstition that "a gun fired over a corpse thought to be lying at the bottom of the sea or a river, will by concussion break the gall bladder, and thus cause the body to float." The superstition evidently found its way from England to the U.S., and is mentioned in Edgar Allen Poe's 1842 story The Mystery of Marie Rogêt (thanks to Ninjagrapefruit on The Straight Dope Message Board for calling the Poe mention to our attention.)

Does it work? Of course not. Well then, what makes a corpse float or sink in water? Several factors:

Lungs. Lungs are like a sponge. When someone drowns, the air sacs in the lungs fill with water. Since a body without air in the lungs is denser than water, it sinks. A person who is killed on the surface and then put in the water tends to float, since the lungs are still full of air (although see below). That's how pathologists can tell whether a person was drowned or was dead before hitting the water.
Body position. A person who was dead before entering the water can still sink depending on the position of the body. If the body is upright when dumped into the water, water can enter the lungs while air escapes. Hence, the body sinks after a short time. If the body is prone (face down), the air in the lungs can't escape, so the body floats.
Body fat. Body fat is less dense than water. The fatter a person is, the more buoyant the body. Muscle on the other hand is denser than water, so people with a lot of muscle--or people who are just plain lean--tend to sink.
Clothing. Some fabrics trap air well; others don't. Natural fibers, like cotton and wool, absorb water and so tend to sink. An exception is silk, whose fine fibers can be woven tightly enough to trap air. Some synthetic fibers, like nylon and polyester, usually don't absorb water, so they can trap air. Other synthetic fibers, such as orlon and other fibers used in cold-weather clothing and sleeping bags, are manufactured like some pastas, with one or more tunnels in them. These tunnels are designed to trap air and insulate the wearer. These fibers float until they fill with water, at which point they sink like a stone.
Putrefaction. A body that sinks doesn't necessarily stay sunk. As the corpse decays, it generates gases that collect in various body cavities. That's why corpses become bloated, whether in the water or not, giving us the charming term "floater."
The water can also affect buoyancy:

Salinity. Probably the most important factor is salinity. Salt water is denser than fresh water, so corpses tend to float in the ocean but not in a river.
Temperature. The water's temperature affects its density. Water is densest at 4 degrees Celsius (about 39 degrees Fahrenheit). Bodies of water often have a thermocline (a depth where there's a dramatic change in temperature). Warm waters "float" and circulate by convection when warmed by the sun; colder waters "sink" and tend to stay there. Hence, there is a slight (very slight) tendency for bodies in cold water to float, since they may be less dense than the water surrounding them.
Can a burst gall bladder have any impact on floating or sinking? Yes, although not necessarily in the way you might think. More important than whether the gall bladder ruptures is whether the skin is broken. If the bladder rupture is strictly internal, there's no effect on buoyancy, since the body's overall density remains unchanged. However, if the skin is broken and the bowels are allowed to come loose (liking that visual, are we?), the body's density may increase. If water enters the body and air and other gases escape, there's a greater chance of sinking. So a burst abdomen can have an effect opposite to what the superstition alleges.

Could firing a cannon over the water cause the gall bladder to burst? Not likely, but it could cause a concussive effect that jarred loose a body snagged in weeds and whatnot on the bottom. So firing a cannon might raise a body, although not for the reasons that the superstition gives.

In The Annotated Huckleberry Finn, Hearn observes, "Once when he was thought to be drowned, young Sam Clemens witnessed a similar scene as the townspeople of Hannibal fired cannon over the water to raise him to the surface. 'I jumped overboard from the ferryboat in the middle of the river that stormy day to get my hat,' he recalled in a letter of February 6, 1870 (Mark Twain's Letters to Will Bowen, 1941, p. 19) 'and swam two or three miles after it (and got it), while all the town collected on the wharf and for an hour or so looked out across . . . toward where people said Sam Clemens was last seen before he went down.'"

Answer 2

I noticed that cherry bombs exploding in a pond or river will cause the fish to rise the top...belly up. I'm not sure of the physiology of the whole thing...and whether or not that relates to human bodies....