The Wine Pages: Homebrew winemaking
Specific gravity a clever sounding word that essentially means density. Density of wine relates to the amount of sugar dissolved in the wine.
Gravity is measured in degrees. A device for measuring gravity is called a hydrometer. Water has a gravity of 1000 degrees. Obvious isn't it?
The gravity of wine increases as you add sugar. The gravity falls as the yeast eats the sugar. The alcohol produced is less dense than water. Your finished wine should have a gravity of somewhere between 1010 and 990. This means pleasently sweet and pretty dry respectivly. If your wine falls below 990 it tends to become so dry it doesnt bear thinking about.
The amount of sugar to add is a very personal thing, and is discussed in the link.
A hydrometer can be obtained for a few pounds from most wine making shops, and of course from Ebay. Most cheap hydrometers are not the most accurate things in the world, so sometimes it is worth calibrating with plain water or a solution of known gravity. For example 12g of sugar in 88g of water would be 1048.
Glass hydrometers are quite fragile and tend to roll off the worktop and smash on the ground. Always keep a spare - if your hydrometer breaks during a wine making session it can be very annoying.