of touch deceives us; when we touch a body hotter than the hand we receive heat from it-when we touch one colder than the hand it receives heat from us; but experience tells us that all the things in a room, when measured by a thermometer, have an equal temperature, yet they do not feel equally so to the hand.

The different degrees in which bodies conduct heat have been ascertained by experiment; air and gases, when confined, are very bad conductors: metals varying in degree among each other are good ones-generally the more dense the body the better conductor it is.

Porous bodies are bad conductors, as are any bodies which contain air confined in cells, such as the feathers of birds-the fur of animals-the bark of trees. All these, how beautiful a provision for the preservation of animal and vegetable life!

Then, again, straw, reeds, &c. are bad ones; so that a thick covering of thatch is a much better covering for a cottage, so far as warmth in winter and coolness in summer are concerned, than either tile or slate.

Tile, being rather a thick and a porous substance compared with slate, is better than the latter; and every one who is in the habit of visiting the cottages of the poor will have observed that the bedrooms of those covered with slate are in the summer extremely hot, and in the winter equally cold.

Slate, again, would be better than iron.

The teacher would do well to observe the variety of fur and hair in animals, varying with the climates they inhabit; in warm climates the hairy coat of animals being short and thin, in the colder ones becoming thick and woolly. The birds of colder regions, that live in the air, have a much greater quantity of plumage than those of the warmer ones; water-fowl, such as ducks, geese, &c., have the interstices between their feathers filled up with down, more particularly on the breast. In the cold weather in winter the birds may often be seen shaking and ruffling up their feathers in order to increase the quantity of air among them, which, being a bad conductor, helps to keep them warm.

Earth is a bad conductor, and the sharpest frosts in consequence scarcely ever get more than a few inches deep into the ground. The temperature of the earth, a very little below the surface, is the same in every climate.

In covering up a potato-pit for the winter, the lighter the soil, and the more of a covering of straw or leaves between it and the potatoes, the better they will be preserved. When it is said the frost gets to the potatoes, the thing really meant is that, the temperature of the air becoming lower than freezing point, the surface covering of the potatoe-pit first gives out heat to the air, then that nearest the surface to the particles adjoining, until, last of all, the potatoes give out heat to what is resting upon them, and so the water of the potatoes gets cooled below freezing and becomes solid, and the potatoe spoiled; hence the necessity of covering them with bad conductors-not to make the soil over them a solid, but as light as possible.

On the same principle, a covering of snow is a great protection, in very severe frosts, to the more delicate plants; although the temperature may be very far below the freezing point, and, in some climates where the cold is great, the thermometer is even down to zero, yet the temperature of the ground, under a covering of snow, would be very little below freezing. Thus water in pipes below the surface, and in springs, is never frozen. In the winter, to prevent water freezing in pipes which are above ground, they are wrapped round with straw or some bad conducting substance, &c. Icehouses with double walls-rooms with double windows, are all instances of the same kind. The application of a kettleholder, having wood or ivory handles to teapots made of metal, &c., belong to the same principle.

The following, by way of a lesson on one of the metals, iron, with the experiments which follow, will convey some idea to the teacher of the mode of proceeding here, and may serve as a model for the way in which he would treat the other metals:

Iron-found in the earth as a mineral-how obtained from the ore?-is a metal, a solid?-can it be made fluid? Yes, sir, by great heat.- Have you ever seen it fluid?—At the little foundry at the blacksmith's shop.-How does it become solid again? By cooling.-What effect has heat upon metals ?* It expands them, makes them longer-it would make an iron ring larger. Have you ever seen this property of expanding by heat turned to a useful purpose? Yes, sir; the village blacksmith

*The difference between the heat of summer and winter will cause such a variation in the length of the ordinary seconds pendulum as to

it;

hooping wheels; he makes the hoop a little too small, heats it red hot, which makes it larger, and it just fits the wheel-he then pours water upon it immediately contracts and makes the joints of the wheel close up and crack, and so it fits tightriveting bolts,* &c.-the experiment of iron bars bringing the opposite sides of a building to an upright position from leaning outwards.

The teacher will point out the various uses to which iron can be applied-how useful from its extending under the hammer-welding (which most other metals do not), and other properties. What is welding? Heating two pieces of iron to a very great heat (called a white heat), then placing them together on the anvil, and beating them with a hammer, they unite as one piece ;-silver and gold will not do this. Platina

welds.

Cast-iron-melted and run into a mould for shape, for grates, saucepans, boilers, tea-kettles, parts of the plough, rollers, door-latches, gate-latches.

Did you ever in winter, in frosty weather, find out that it was colder to the hand to touch iron than wood? Yes, sir.-Why? Do not know, sir.—Teacher (making the children touch substances of different conducting powers, a piece of marble, stone, wood, wool, flax, cotton, &c., pointing out to them that all have the same temperature as the room, which is below that of the hand, and ought, so far as this is concerned, to affect it equally) Because iron is a better conductor of heat than wood or any of the others; being very cold in frosty weather, and much colder than your hand, it carries away the heat much more rapidly than wood, and it has very little to give back in return this rapid loss of heat causes a very unpleasant sensation; if you hold the iron long enough, it will get the same degree of warmth as the hand, and the unpleasant effect will affect its time of vibration; and in the building of iron bridges, allowance is obliged to be made for what is called the play of the iron, between summer and winter heat, or the whole would come down, and I believe in some of the large tubular structures of iron lately erected over rivers, allowance has been made for the unequal expansion of the metal on the sunny and shady sides.

* I am told, in testing the anchors in the Dockyard at Portsmouth, that the largest anchors having a strain on them of perhaps 150 tons, and being in length about 30 feet, and as thick as the body of a man, that immediately the strain is taken off they will collapse as much as an inch, and that this shrinking is visible to the eye of a looker-on.

cease; the stream of heat from the iron to the hand, and the hand to the iron, will exactly balance each other; that is, the two substances, your hand the one, and the iron the other, will then impart equal heat to each other.

They may also be told to touch the different substances, marble, wood, stone, iron, &c., with their lips, which, as they are much more sensible to cold, will point out to them more strikingly how much the sense of touch deceives them.

Experiment. The teacher, taking a polished cylindrical piece of iron, with a piece of white paper held tightly over it, holds it in the flame of a candle, and observes it does not charthe same on a piece of wood, and, exposing it in the same way, it immediately turns black;-the iron being so good a conductor does not allow the heat to rest with the paper, but immediately takes it away, &c.; the wood not conducting it so rapidly causes the paper to burn.

On this principle water may be made to boil in a paper kettle, or in an egg-shell-when boiled away, both substances would immediately burn.

Experiment. Metallic rods of equal lengths and substance, one of each smeared with bees' wax, and immersed in a heated fluid, the heat travels along each rod, from particle to particle,* and the one on which the wax melts first is the best conductor, the one on which it next melts the second best, and so on— the order in which the wax melts being the order in which the rods conduct the heat.

Glass-a solid, can be softened by heat, so as to be drawn out into a fine thread-allows light to pass through it; in what way does man turn this property to his use? Windows, lanterns, spectacles, telescopes, &c.; does not allow the heat of the fire to pass through it-the heat of the sun does.What other substances allow light to pass through them? Water, horn, air, &c.

* The following experiment, which is easily tried, shows the way in which a fluid, as water, is heated by a flame placed under it: take a glass tube, open at one end, and about an inch or so in diameter; pour water into it, so that there may be a column of several inches in length, and place it over a spirit lamp. As the flame heats the water, drop sand into it, and a double current will be observed, one downwards along the sides of the vessel, the other upwards through the centre of the fluid: apply the heat to the surface on the sides of the vessel, and the currents will be reversed. The reason of all this to be pointed out.

Why will a glass sometimes break by pouring hot water in it? Answer: Solids convey heat from particle to particle, and some solids do this more slowly than others; glass conveys it very slowly, and the hot water in contact with the inner surface causes the inside surface of the glass to expand, but the outer one, not being so hot, will not follow it, and so snaps, being very brittle. Thin glass will not break so readily as thick, the distance between the two surfaces being smaller, the heat gets through sooner, and the inner and outer surface are almost instantaneously raised to the same temperature—hence chemists use thin retorts.

On the subject of Metals used for the various purposes of social life, the class of teachers for whom these pages are intended may give a great deal of useful instruction.

They might draw attention to the different ores, showing specimens of them, and mentioning the kinds of earths and other substances with which they are generally mixed—where found in our own and other countries-the per centage of metal found in an ore, in one case making it what is called a rich one, in another so small as to scarcely make it worth working anything peculiar in the way in which metallic veins run, not being stratified, &c.-depth of mines-the number of workmen employed in the mining of any particular ore, the method and necessity of transporting it from the place where it is found for the purposes of smelting, either from the people not knowing how, or for want of coal, &c.—great inconvenience of this in a commercial point of view, from having to transport so large a proportion of the ore which is useless*

* "When a mass of matter is to be removed, a certain force must be expended; and upon the proper economy of this force the price of transport will depend. A country must, however, have reached a high degree of civilization before it will have approached the limit of this economy. The cotton of Java is conveyed in junks to the coast of China, but from the seed not being previously separated, three quarters of the weight thus carried is not cotton. This perhaps might be justified in Java by the want of machinery to separate the seed, or by the relative cost of the operation in the two countries. But the cotton itself, as packed by the Chinese, occupies three times the bulk of an equal quantity shipped by Europeans for their own markets. Thus the freight of a given quantity of cotton costs the Chinese nearly twelve times the price to which, by a proper attention to mechanical methods, it might be reduced."-BABBAGE on the Economy of Machinery.