"There are no tides in lakes because they are generally so small that when the moon is vertical she attracts every part of them alike; and, therefore, by rendering all the waters equally light, no part of them can be raised higher than another. The Mediterranean and Baltic Seas suffer very small elevations, because the inlets by which they communicate with the ocean are so narrow that they cannot, in so short a time, receive or discharge enough to raise or sink their surface sensibly.
"Air being lighter than water and the surface of the atmosphere being nearer to the moon than the surface of the sea, it cannot be doubted that the moon raises much higher tides in the air than in the sea. Therefore many have wondered why the mercury does not sink in the barometer when the moon's action on the particles of air makes them lighter as she passes over the meridian. But we must consider, that as these particles are rendered lighter, a greater number of them are accumulated, until the deficiency of gravity is made up by the height of the column; and then there is an equilibrium, consequently an equal pressure upon the mercury as before; so that it cannot be affected by the aerial tides. It is probable, however, that stars seen through an aerial tide of this kind will have their light more refracted than those which are seen through the common depth of the atmosphere; and this may account for the supposed refractions of the lunar atmosphere that have been sometimes observed.
"You see now how the tides are caused; while there may be some influences at work other than those exerted by the sun and moon, the latter are the chief ones, so I will not attempt to explain any other.
"Here, on the Passaic River, we do not have excessive tides, as the highest on the coast near us seldom rise over ten or twelve feet. As a rule, tides rise highest and strongest in those places that are narrowest. In the Black Sea and the Mediterranean, the tides are scarcely perceptible, while at the mouth of the Indus, in the Bay of Fundy, and other places, they rise thirty or more feet at times. The general rise, however, in mid-ocean, is from eleven to twelve feet.
"The diameter of our moon is nearly 2,200 miles, and her distance from the earth is about 240,000 miles; so you see it is not her size, but her proximity to the earth that gives her so much influence over the tides; for the sun, which is many times larger than the earth and moon combined, because of its being some ninety-three millions of miles away, exerts only one sixth of the attraction on the earth that the moon does.
"These facts, children, should be remembered, as you may often be called upon to make use of them.
"Oh, papa!" said Jessie "how many wonderful things there are in this world."
"But I have not told you all, my dear. There is much more to learn, but I hope the knowledge you have now acquired will act as an incentive, and cause you to pursue this study further."
