The Jolly Jug was popular in the early to mid 19th century, and no doubt was thought a great party joke, especially in the days before television! Elsmore & Forster in Staffordshire were apparently one of the firms that manufactured such items.
When you fill the jug with water, or with wine, you can pour from the spout in the normal way, but you can also make the liquid come out through a hole in the base of the jug. When you are pouring from the bottom of the jug, you can also make the flow stop, apparently at will. Hours of endless fun!
Jolly Jugs, or siphon jugs as they are sometimes called, have a simple siphon inside the handle. Well, simple as far as the science is concerned. But very impressive to be made in pottery. Inside the jug is a hole which connects to a tube that runs up one side of the handle. If you look at the underside of the jug, you see a second hole, and this hole connects to a separate tube that runs up the other side of the handle. Towards the top of the handle the two tubes meet. There will also be a small air hole at the top of the handle, just a couple of millimetres across.
To make the siphon action work, you need to tilt the jug back slightly, keeping your finger over the little hole in the handle. Initially there will be liquid in one of the tubes in the handle - the tube that connects to the hole INSIDE the jug. When you tip the jug back slightly, the liquid will move up that tube and down into the other tube that connects to the hole on the underside of the jug. You now have a working siphon, and liquid will continue to flow from the bottom of the jug until you remove your finger from the little hole in the handle. This will break the siphon, and the liquid flow will cease.
For the trick to be most effective, the jug needs to be fairly full, so the the backward tilt is not too obvious. In addition, if you remove your finger from the hole with only a slight movement, the audience will not be aware that you have done anything, and will be puzzled as to how you can stop the flow of liquid apparently at will!