But this requires a little explanation.
It is rather bold in me, you may think, to assert so freely, that all the year round, from one end of the earth to the other, the human body is never colder nor hotter than mine is, for instance, at this present moment. "Hot" and "cold" is soon said, you argue: but the exact varieties of more or less are not so easy to measure, and especially not easy to remember, with reference to so many bodies, scattered over the face of the whole earth. What may be warmth for one in one case, may not be equal warmth for another; and even supposing that the same individual learned man could go and inspect every part of the globe in succession, how could he possibly recall, while touching the body of a negro in Senegal, in July, the exact amount of animal heat he had found in a Greenland Esquimaux in January?
Be content. I should not have settled the question so cavalierly, if people had not discovered an infallible method of estimating accurately, and always in the same manner, the degree of warmth, in other words, the temperature of the body.
Let us first see, then, what this method is, though it will oblige us to digress a little; but you are accustomed to that now, surely; and besides, if I were to go straight ahead, you would not be able to follow me.
Do you ever recollect being very cold? Let mammas look after their little girls as much as they please, to prevent it, it is sure to happen to every one some day or other. Now does it not seem at those times as if the whole body were contracting itself—and when people are shivering with cold, have they not a shrunk, shrivelled look? When the weather is very hot, on the contrary, our bodies feel as if they were swelling and stretching, and one seems to take up more room than before. This is the case with all bodies. Heat swells, or, as learned people call it, expands, them: cold shrinks or contracts them. Furthermore, mercury is one of the things most susceptible of this action of heat and cold, and we have had recourse to it accordingly, in the construction of the thermometer, [Footnote: Thermometer comes from two Greek words: thermos, heat; and metron, measure. The degrees in the Thermometer about to be described are marked on the Centigrade principle. [Not the one (Fahrenheit) in general use in the United States.] a very useful instrument, which you will hear spoken of all your life.
The thermometer, or heat-measure, consists of a little hollow ball filled with mercury, out of which rises a small tube of very thin glass, in which the mercury can move up and down. When the thermometer is exposed to heat, the heat causes the mercury to expand, so it goes up the tube; when the thermometer is exposed to cold, the mercury contracts and sinks again.
Now suppose you were to melt some ice in the palm of one hand, and try to dip a finger-tip of the other in a saucepan of boiling water; you would find a great difference of temperature between the two, would you not? Which difference of temperature people have succeeded in measuring with the thermometer, as accurately as your mamma measures a piece of cloth with her yard measure.
This is how it is done:
You surround the ball of mercury with pounded ice, and while it is melting make a mark at that point in the tube where the mercury has stopped in its descent. Then plunge the thermometer into boiling water. Whereupon the mercury goes up, up, up, till at last it reaches a point beyond which it will not pass. Here a second mark is made, and the space between the two marks is divided into a hundred perfectly equal parts, indicated by so many small lines, which are called degrees. But this word degrees has a double meaning in some languages. It means steps as well as the degrees of measurement we are talking about; steps being, as you know, the perfectly equal parts into which a staircase is divided. Fancy the mercury-tube a staircase, then, rising from the cellar where the melting ice is, up to the garret where the boiling water is, and let it consist of 100 steps. The mercury goes up and down this staircase, according as the temperature it encounters approaches that of the boiling water or of the melting ice; and if you wish to know exactly how far it is from the cellar or from the garret, you have only to count the steps. Hence arise those expressions which you so often hear—high temperature and low temperature. These mean, temperature according to which the mercury goes up or down this staircase.
On the actual floor of the cellar where the ice melts, there are yet no degrees (a floor is not a step, you know), so there you find the word zero, which means a cipher or nought. Then you begin to count 1, 2, 3, 4 degrees, marked by lines up to 100, where you reach the garret, i.e. the boiling-water height.