nor the other much depressed below, the general surface of the waters. The sun has, however, a very sensible effect on the waters, and increases or diminishes the tides, as it acts in conjunction with, or in opposition to, the moon. EMILY. I do not quite understand that. MRS. B. The moon is a month in going round the earth; twice during that time, therefore, at full and at change, she is in the same direction as the sun; both then act in conjunction on the earth, and produce very great tides, called spring-tides, as described in fig. 4. at A and B: but when the moon is at the intermediate parts of her orbit, the sun, instead of affording assistance, weakens her power by acting in opposition to it; and smaller tides are produced, called neap-tides, as represented in fig. 5. > EMILY. I have often observed the difference of these tides when I have been at the sea-side. But since attraction is mutual between the moon and the earth, we must produce tides in the moon; and these must be more considerable in proportion as our planet is larger. And yet the moon does not appear of an oval form. MRS. B. You must recollect that, in order to render the explanation of the tides clearer, we supposed the whole surface of the earth to be covered with the ocean; but that is not really the case, either with the earth or the moon, and the land which intersects the water destroys the regularity of the effect. CAROLINE. True. We may, however, be certain that whenever it is high-water the moon is immediately over our heads. MRS. B. Not so, either; for as a similar effect is produced on that part of the globe immediately beneath the moon, and on that part most distant from it, it cannot be over the heads of the inhabitants of both those situations at the same time. Besides, as the orbit of the moon is very nearly parallel to that of the earth, she is never vertical but to the inhabitants of the torrid zone; in that climate, therefore, the tides are greatest, and they diminish as you recede from it and approach the poles. CAROLINE. In the torrid zone, then, I hope you will grant that the moon is immediately over, or opposite to, the spots where it is high-water. MRS. B. I cannot even admit that; for the ocean naturally partaking of the earth's motion, in its rotation from west to east, the moon in forming a tide, has to contend against the eastern motion of the waves. All matter, you know, by its inertia, offers some resistance to a change of state; the waters, therefore, do not readily yield to the attraction of the moon, and the effect of her influence is not complete till three hours after she has passed the meridian, where it is full tide. EMILY. Pray what is the reason that the tide is three quarters of an hour later every day? MRS. B. Because it is twenty-four hours and three quarters before the same meridian on our globe returns beneath the moon. The earth revolves on its axis in about twenty-four hours: if the moon were stationary, therefore, the same part of our globe would, every twenty-four hours, return beneath the moon; but as, during our daily revolution, the moon advances in her orbit, the earth must make more than a complete rotation in order to bring the same meridian opposite the moon, we are three quarters of an hour in overtaking her. The tides, therefore, are retarded for the same reason that the moon rises later by three quarters of an hour every day. We have now, I think, concluded the observations I had to make to you on the subject of astronomy. At our next interview, I shall attempt to explain to you the elements of hydrostatics. FLUIDS AND LIQUIDS. -OF NON-ELASTIC FLUIDS, SCARCELY SUSCEPTIBLE OF COMPRESSION.-OF THE COHESION OF FLUIDS.-OF THEIR GRAVITATION.OF THEIR EQUILIBRIUM. OF THEIR PRESSURE. OF SPECIFIC GRAVITY.-OF THE SPECIFIC GRAVITY OF BODIES HEAVIER THAN WATER. OF THOSE OF THE SAME WEIGHT AS WATER. OF THOSE LIGHTER THAN WATER. OF THE SPECIFIC GRAVITY OF FLUIDS. MRS. B. WE have hitherto confined our attention to the mechanical properties of solid bodies, which have been illustrated, and, I hope, thoroughly impressed upon your memory, by the conversation we have subsequently had on astronomy. It will now be |