carrying heat from the hot chamber to the cold. These are the essentials. The cranks, and wheels, and cylinders, and pistons, are conveniences, but they are not really essential; we can conceive of an engine without them.

2. Let us take an ordinary low-pressure engine. Here we have a boiler and a condenser, and as the engine works the heat is carried from the boiler to the condenser. Now, in order to show you that this is the case, and that the heat must be carried from a hot substance to a cold one, let us consider an engine of quite a different kind,—I mean an ordinary fire or flue, for an ordinary fire is to some extent an engine. You have in it the production of mechanical effect in the process of carrying heat from a hot substance to a cold. In an ordinary fire we have the hot air rushing up the chimney from the fire, and mingling with the cold air above—that is, we have the carrying of heat from a hot substance to a cold; the hot current of air rushing up the chimney and mingling with the cold air outside. And an engine of this kind-for it is an engine-really produces work, and you might take advantage of the current of air that is going up the chimney; indeed, an ordinary fire is a ventilating engine, and will do very often the work of ventilation that would otherwise require an ordinary engine.

3. Now let us consider our own earth as a heat engine. You all know that the earth is a body which revolves round its axis in twenty-four hours, and that the equator, or the central part between the two poles of the earth, is that particular region on which the sun shines very directly whereas, on the other hand, the poles are the regions where the sun hardly shines at all. The equatorial regions are the boilers of this heat engine, and the pole is its condenser; and this engine-for it is an engine-works in the process of carrying hot air from the equator to the cold parts.

4. What takes place? The sun shines directly upon the equator, and this heated air, just as in the case of an ordinary fire, ascends to the top of the atmosphere, and is replaced by cold air blowing in from the poles. We have a cold current of air that blows in from the poles to the equator along the ground; and on the other hand the heated air finds its way in the upper regions of the atmosphere from the equator to the poles. If this heat engine were at rest, if it did not revolve, we should have a cold north wind blowing along the ground from the pole, and a warm south wind blowing from the equator towards the pole; but,

inasmuch as it revolves on its axis in twenty-four hours, instead of a north wind blowing along the surface, you have a north-east wind, called the “trade wind;” and, instead of a south wind, you have a south-west wind, called the "anti-trade wind." Here, then, you have a series of winds blowing, by means of which heat is really carried from the hot parts to the cold parts of the earth.

5. There is still another way in which the earth acts as a heat engine, and that is this: We have, high above the earth, a stratum of air extremely cold, and heat is constantly being carried from the warm moist air next the ground to this cold air next the cold clear sky. We have ascending currents carrying up this warm moist air into the upper regions: and very often this process cools the air to such an extent that it can no longer retain all the aqueous vapour, and this aqueous vapour is deposited in the shape of rain, hail, or snow, and this ultimately finds its way to the ground; so that, just as in an ordinary engine, you have the cold water pumped back from the condenser to the boiler, for here the ground acts as the boiler and the upper regions as the condenser.

6. Thus you have an arrangement by which heat is carried from the warm moist air next the earth to the cold air next the cold clear sky; and in this respect also the earth acts as a heat engine. Now the earth being a heat engine does work, and the seaman who hoists a sail, or the miller who grinds his corn by means of a windmill, really take advantage of the work done by the earth engine, as much as we do when we have work done by a low-pressure engine or a locomotive.

7. But again in the ordinary heat engine there are two things that keep the engine working smoothly. First of all we have the "governor" that regulates the supply of steam; and then we have the "fly-wheel" that equalizes the rate of motion of the engine; and by means of these two things the engine is kept working smoothly. But in the earth engine these arrangements appear at first sight to be entirely wanting; it seems to be an engine that works by fits and starts. For a long time we have a period of profound repose; then all at once you see that a storm is brewing, and especially in the regions next the equator these storms become terrific tempests, doing great damage, throwing down houses, tearing up trees, and devastating whole districts; while at sea they produce numerous shipwrecks. In such cases what is the matter with this earth engine? Is it not properly

regulated? Is it gone mad? If so, let us endeavour to see if, at any rate, there be not some method in its madness.

1. In order to do this, we shall mount, if you please, into the celestial regions, and consider a still greater heat engine than even our own earth-I mean our sun-for the sun is also a heat engine. The sun is so far away that if a railway train at full speed were to go from the earth to the sun, it would take between 200 and 300 years to reach the sun; and it is so large that the same train would take about nine years to go round the surface of the sun. So you may imagine how large the sun is. The sun, like the earth, revolves upon its axis, but only once in 25 or 26 days. Well, I said that the sun was a heat engine. But in the first place I do not think there is any clear evidence of a system of polar and equatorial currents in the sun similar to those on the earth.

2. But although with regard to the sun we have no direct evidence of a series of currents of wind going from the sun's poles to the equator and back again, yet, on the other hand, we have very strong evidence of a series of ascending and descending currents going up and down in the sun's atmosphere, similar, no doubt, to those ascending and descending currents in the atmosphere of the earth. For, first of all, the temperature, or heat of the sun is very much greater than the heat of the earth. You have in the sun a very hot substance, and a cold clear sky surrounding the sun as we have surrounding the earth.

3. In the next place, let us think for a moment what it is that

makes heated air go up. It is because this heated air becomes lighter; that is to say, it has less weight. Now, what does weight mean? It means the attraction of the earth. Without the attraction of the earth you would have no weight; and if you had an earth twice as heavy as this you would have double the attraction. Hence, at the surface of a large body like the sun the attraction is enormously greater than it is at the surface of the earth; and, consequently, if a thing gets lighter on the sun, it rises much more rapidly than the same thing would do on the surface of the earth, because the attraction is so very much greater. If there were no attraction, there would be no up-anddown motion at all; but when the attraction is very great, then the up-and-down currents are extremely strong.

4. On these two accounts, therefore-both on account of the great difference of temperature between the surface of the sun and the cold sky about it, and on account of the great weight of the sun-these ascending and descending currents on the sun's surface are extremely strong. I should surprise you if I were to mention how strong they are. They move at the rate of something like 30 or 40 miles in a second at least. That is very

much stronger than the strongest hurricane on the earth's surface. Well, as these ascending currents go up in the sun just like the currents on the earth, they carry with them a quantity of vapour; I do not mean to say aqueous vapour, but very likely the vapour of heated metals.

5. As these vapours ascend, they get into a colder region, and the vapours are deposited in the shape of clouds, just like the clouds of the earth's atmosphere, and they find their way down again to the sun's surface. Now, remember, that the sun shines by its own light, and consequently these cool vapours that are falling down in the shape of celestial rain or hail will appear darker than the bright parts of the sun that are not cool. If this be true, we ought to expect that when the sun is seen by a powerful telescope it should present a mottled appearance, consisting of black patches upon a bright ground. Now, over all the surface of the sun you have this mottled appearance.

6. You thus see that precisely the same thing is going on in the sun as in the earth, in the way of ascending and descending currents, only very much more powerfully. The ascending currents convey metallic vapour, and these are deposited when condensed by the cold, and find their way down as clouds, which appear as black patches when you examine the sun very closely.

There is one thing I ought to mention. A storm of hail or rain on the earth is stopped by the surface of the earth; but the sun is so hot there is no solid or liquid surface. Very likely the sun is one mass of gas, and the surface that we see, instead of being solid or liquid substance, is one of cloud.

7. Thus there is a number of bright patches and a number of black patches. No doubt these bright patches are the hotter parts of the sun, and these black patches denote something like solar rain, or matter that has been cooled by contact with the upper regions of the solar atmosphere. Now let us imagine ourselves traversing the cloudy surface of the sun, and what shall we see? Perhaps, as we travel over the solar surface, we may come to the brink of a most enormous chasm; a chasm so enormous that, as far as surface extent is concerned, it might swallow up twenty or thirty worlds like our earth, not, perhaps, so deep, but, at any rate, something like 3,000 or 4,000 miles deep. This chasm would have sloping sides that are somewhat less luminous than the sun's surface, and it would have a bottom still darker than the sloping sides; in fact you would have three gradations of light-the bright surface of the sun, the sloping sides of the cavern (less bright), and you would have the black bottom of the cavern that was still less bright. This is, what is called a sun spot."

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8. Thus, there must be enormous storms taking place on the surface of the sun-this sun-engine, as it were, is always working very powerfully and very strongly; and no doubt in the case of the sun spot we see the cooled matter or cloud which is falling down; and as I told you that the sun, unlike the earth, is not a solid or liquid surface, but merely cloud, you can imagine that this shower goes down a long way before it gets into the hot regions and becomes melted. I have no doubt that is an explanation of the fact that the bottom of the spot is far below the surface of the sun. Well, these great disturbances on the sun's surface, or sun storms, take place in the equatorial regions, just as in the equatorial regions of the earth we have great cyclones and hurricanes. These sun storms which give rise to the sun spots occur near the equator, and do not take place at the poles at all. Here, then, is a curious likeness between the sun and the earth. And now comes, perhaps, the strangest part of the whole thing. If we notice the sun from year to year we find that some years there are very few of these spots or storms in the sun, and some years there are a great many. A record of