It is a common enough thing among us for anyone who feels indisposed during the dog days of summer to visit a spa. I can say that for several years, thank the Lord, I have had reasonable health even though occasionally feeling a little sluggish and out-of sorts; but as soon as I arrived in the mountains I received, as it were, new life and it was as if a heavy burden had been lifted from me. Now that I had been in Norway a few days, however, even though I had by no means debauched myself, I was again feeling somewhat sluggish, but I revived as soon as I returned to the mountains. Without doubt, the pure and breezy atmosphere contributed a good deal to this. It is commonly thought that the air is thinner in high places and that, for that reason, people have to take wet sponges and hold them in front of their noses in order to breathe through denser air. Were it not that the barometer showed me that the air pressure is lower there, I should have said that this was simply because they have been walking uphill and have become breathless because of the blood pressing on the lungs – just as in the case of someone who causes an acceleration of the circulation of the blood by running or some other brisk exercise. It seems to me, however, to run counter to reason. We know that these mountains are higher than all the other hills in the world because there is not a single river that runs over them; the western rivers run into the western sea and the eastern rivers run into the eastern sea. If we then take into consideration all the waterfalls and cateracts that lie between the mountains and the sea, we must conclude that the mountains have a considerable height both on the seaward and on the landward side. Now, when the wind passes from the land onto the sea and has to cross over this very considerable height, it follows that it must be compressed (which is the reason why it gets both stronger and colder). If it is compressed it must become denser and consequently it often freezes here even in the height of summer. Cold consists of the air being compressed and warmth consists of it being rarefied, the consequence of this being that the air is not thinner in the mountains.