time, will be the first to evaporate and feel the cold produced, in consequence. Evaporation, under favorable circumstances, will produce cold enough to form ice. Belzoni, the famous traveller, conscious of this fact, undertook, with the utmost confidence, to show the sultan that he could turn water into a solid, notwithstanding he was threatened, if he failed, with the loss of his head. He, of course, succeeded; making use of the now familiar means of an air-pump, in the receiver of which a little water being placed, and a vacuum produced, a film of ice soon made its appearance, much to the surprise of the sultan, who in those days was not familiar with the frozen luxury of the Wenham and Rockland lakes, which the enterprise of our countrymen has since diffused over the whole world. It is usual for the people of warm climates to cool their drinking-water by keeping it in porous earthen jars, or wrapping the vessels which contain it, in moistened cloths, both of which, especially when exposed to a draught of air, favor evaporation, and consequently produce a very considerable cold.

There are two other processes, called by men of science "conduction" and "radiation," by which cold is produced both in liv.

ing beings and lifeless objects; and while this is going on, heat is by the same means conveyed from these to other living beings and lifeless objects; but just now it is the cooling effects only that will be explained.

By conduction we are to understand that process by which heat* passes through bodies, living or otherwise; and it is common to call such bodies "good conductors" and "bad conductors," according to the ease and quickness with which they allow of its passage. If any one takes an ordinary pin or bit of wire, and, holding it by one end, puts the other into the flame of a candle, he will soon find it too hot to hold, and be forced to drop it, for the heat will pass almost instantaneously from the blaze to the fingers, and

* When speaking of heat in this little book, it is to be understood in its popular sense, as referring to the effects produced by it on the senses, and not to its definition by science, according to which heat is a form of force; and its various manifestations, of which cold even is one, are only its changes, the nature of which is still in a great degree mysterious to the philosopher, and, of course, does not admit of any successful attempt at explanation to the child. When, with the advancement of years, he reaches the higher studies of chemistry and natural philosophy, the occasion will come for the proper consideration of the subject of heat in its scientific aspects.

burn them. The pin or wire is therefore called a good conductor of heat, or the iron of which it is made—as is, in fact, every metal, whatever form it may have. If, instead of the metallic thing, a bit of paper is held to a flame, it will take fire and burn to the very tips of the fingers without their feeling the least sense of warmth. Paper is therefore called a bad conductor, and a great many substances of different kinds have the same character.

Wood, ivory, cloths, furs, feathers, and other familiar substances, are bad conductors, and, though not generally recognized by this name, are constantly made use of because they have the quality which it expresses. Wooden handles are fixed to the iron poker with which the fire is stirred; a pad of cloth is put on the holder of a flatiron in ironing clothes; no one attempts to take a boiling pot from the fire with the hand until it is guarded with a cloth of some kind; and the parlor tea-urn would burn the delicate dame who serves it if there were not a bit of ebony, ivory, or some other bad conductor, let in between the handle and the body of the vessel which contains the hissing-hot fluid, to prevent its heat from passing to the fingers that grasp it.

Radiation is the process by which heat passes from a body apparently in radii, or rays. Radius, which is a Latin word, means the spoke of a wheel, and the arrangement of the spokes of the wheel passing from the centre to the circumference gives a very good idea of the way in which heat seems to pass from a body of any kind. The rude pictures of the sun seen upon sign-boards, where a round flaming head is represented to be darting spikes of fire on all sides, will supply further illustrations of what is meant by the term "radiation." It is by waves, however, that, as we are told by the philosophers, heat passes from body to body; thus the heat of the real sun reaches us; and it is also in this way that we receive the warmth of the stoves, grates, and other contrivances for heating us. The whole earth, and everything upon it, is ceaselessly radiating heat, and thus there is a constant tendency in nature to produce by this means in all objects, living or otherwise, an equality of temperature; but the effort is checked by other processes, such as the evaporation and conduction, of which we have spoken. The greater coldness of the night is due to the larger degree of radiation from the earth of heat, in proportion to what it then re

ceives from the sun, than during the day. The inhabitants of India make a useful application of this fact. They expose shallow pans filled with water to the night air, so arranged as to favor radiation, and the cold produced is generally sufficient to cover them before morning with films of ice.

Questions.-State the way by which sailors find out the direction of the wind. What is evaporation? What was Belzoni's experiment? What is conduction? Give an illustration of conduction. What is radiation? What is the tendency of the earth and all bodies? How is equality of temperature checked? How is ice produced in India?

CHAPTER X.

How to retain Animal Heat.-Object of Clothing.Good and Bad Conductors. -Winter Clothing.— Proper Clothing in Moist Weather.

DURING cold weather we have, as has been explained, to keep up a brisk internal fire in our bodies. This, we know, is done by taking in a great abundance of carbon, which is furnished by the food we eat, and plenty of oxygen to consume it, which is inhaled with the pure air we breathe. The