It has already been mentioned that air expands when heated. The same rule holds good when applied to other objects, such as the various liquids, metals, &c. A very familiar example of this fact is the “boiling over” of water, when the vessel has been filled too much to allow for the expansion of the heated liquid.
Advantage has been taken of this principle in the formation of the Thermometer, a word which signifies “heat-measurer.” Liquid of some kind is placed in an hermetically sealed tube, generally terminating with a bulb, and in proportion to the heat the liquid expands, and is forced up the tube.
Any liquid will answer to a certain extent, but, as water freezes at 32°, it would be useless for measuring degrees of cold below the freezing point. Coloured spirits of wine are used; but the very best liquid is mercury, which is a metal in a state of fusion.
This expansion by heat is so powerful in iron, that it is utilised in several ways.
Take, for example, wheel-making. The iron tire is made rather smaller than the wheel, and is then placed in a fire until it is red-hot. It then expands so much that it can be easily slipped over the wheel as it lies on the ground. Cold water is then dashed on it, and the tire contracts with tremendous force, binding the parts of the wheel firmly together.
In all buildings where iron is much used, such as iron bridges, iron beams, &c., it is necessary to make allowance at both ends, so as to permit the iron to expand on a hot day and contract on a cool one. Buildings formed of stone and iron were once thought to be safe in case of fire. They are now known to be just the contrary, the stone flying with the heat, and the iron expanding.
USEFUL ARTS.
CHAPTER XII.
The Cassava Press and its Structure.—Mode of using it.—The Siamese Link.—An ingenious Robbery.—Muscles and their Mode of Action.—Human Arms and Steelyard.—Change of Direction.—The Human Hand and Wrist.—Story of a Carpenter.—The Pulley.—Reduction by Friction.—Past and present Engines.—Oiling Machines.—Treatment of the Sewing Machine.—Use of Paraffine.—Disuse of Machine hurtful.—Human Joints.—Synovia and its Value.—Disuse of Joints hurtful.—The Lazy-tongs and its Usefulness to Invalids.—Suggestions for Improvement.—Larva of the Dragonfly and its Mask.—Curious Mode of seizing Prey.—Proboscis of the Housefly, and Mode of using it.—The Apple-parer.—Squirrel and Nut.—Structure of Teeth.—Rock-splitting.—Powers of Ice.—How the Pebble-ridge is formed.—Splitting Stones by Moisture.—The Diamond Drill.—Ovipositor of the Gad-fly.—Curious Similitude of Structure.