_Mrs. B._ This was the system of Ptolemy, in ancient times; but as long ago as the beginning of the sixteenth century it was generally discarded, and the solar system, such as I have shown you, was established by the celebrated astronomer Copernicus, and is hence called the Copernican system. But the theory of gravitation, the source from which this beautiful and harmonious arrangement flows, we owe to the powerful genius of Newton, who lived at a much later period, and who demonstrated its truth. _Emily._ It appears, indeed, far less difficult to trace by observation the motion of the planets, than to divine by what power they are impelled and guided. I wonder how the idea of gravitation could first have occurred to sir Isaac Newton? _Mrs. B._ It is said to have been occasioned by a circumstance from which one should little have expected so grand a theory to have arisen. During the prevalence of the plague in the year 1665, Newton retired into the country to avoid the contagion: when sitting one day in an orchard, he observed an apple fall from a tree, and was led to consider what could be the cause which brought it to the ground. _Caroline._ If I dared to confess it, Mrs. B., I should say that such an inquiry indicated rather a deficiency than a superiority of intellect. I do not understand how any one can wonder at what is so natural and so common. _Mrs. B._ It is the mark of superior genius to find matter for wonder, observation, and research, in circumstances which, to the ordinary mind, appear trivial, because they are common; and with which they are satisfied, because they are natural; without reflecting that nature is our grand field of observation, that within it, is contained our whole store of knowledge; in a word, that to study the works of nature, is to learn to appreciate and admire the wisdom of God. Thus, it was the simple circumstance of the fall of an apple, which led to the discovery of the laws upon which the Copernican system is founded; and whatever credit this system had obtained before, it now rests upon a basis from which it cannot be shaken. _Emily._ This was a most fortunate apple, and more worthy to be commemorated than all those that have been sung by the poets. The apple of discord for which the goddesses contended; the golden apples by which Atalanta won the race; nay, even the apple which William Tell shot from the head of his son, cannot be compared to this! Questions 1. (Pg. 80) Into what two classes are the planets divided, and how are they distinguished? 2. (Pg. 80) By what reasoning do you prove that the sun contains a greater quantity of matter than any other body in the system? 3. (Pg. 81) What two circumstances govern the force with which bodies attract each other? 4. (Pg. 81) Were a planet removed to double its former distance from the sun, what would be the effect upon its attractive force? 5. (Pg. 81) Why would it be reduced to one-fourth?