This is easily said, dear child; but suppose that you do not comprehend it? Well, you need not be ashamed. There is no possibility of comprehending it until one has learnt what RESPIRATION is--so here we are stopped short.

To-morrow, then, when we will begin with the study of this third part of the History of Nutrition; and if the first two have amused you, I feel pretty sure you will not find this last one dull.

LETTER XVIII.

ATMOSPHERIC PRESSURE.

When we have been laboring very hard, my dear child, and want to rest for a minute, we say, _Let us take breath_; because breathing is an action which takes place of itself, requiring neither effort nor attention on our part.

But, if it takes place of itself, it does not explain itself; consequently, when I say to you, _Now, let us take breath_, this is not a signal for my having a rest, for I have undertaken to explain Respiration to you.

If you were a German, I would remind you of what so often happens when you put a fork into a dish of sour-krout. You want to lay hold of a little bit merely, but the strips of cabbage-leaf are twisted one within the other, and hang together in spite of you, so that withoutintending it you get hold of a whole plateful at once.

Now this Respiration affair is something like the sour-krout story--begging your pardon for the comparison. I should have liked to give you only a small plateful--a child's plateful--of it; but I feel the explanations coming, hanging one upon the other; and, whether I will or no, I must treat you like a grown-up person, and we must give up for once the nice little doll's dinners with which we began.

In my opinion, you will lose nothing by the change if you will but pay attention; for about that soft little breath of yours, which is always coming and going over your pretty lips, there are many more things to be learnt than you have heard of yet. As I said just now, you will find you have got hold of a plateful all at once. A good appetite to you!

To prevent confusion we will divide the subject into two parts. I shall explain to you first, _How we breathe?_--a very curious question, as you will see. And afterwards we will examine, _Why we breathe?_-- which is still more interesting.