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full of interest. And although a high degree ofmathematical training is requisite, in order fully to ap-prehend its significance, yet a good general idea ofthe subject may readily be obtained even by those whoare not profoundly versed in mathematics. I proposeto endeavour, in this place, to present the subject in apurely popular, yet exact manner. I wish the readerto see not merely how the law of gravity accounts forthe more obvious features of the moon’s motion, butalso how her peculiarities of motion—her perturba-tions—are explained by the law of attraction. On theone hand the Scylla of too great simplicity is to beavoided, lest the reader should be left with the im-pression that the evidence for the law of gravity is notso complete as it actually is; on the other, the Cha-rybdis of complexity must be escaped from, lest thegeneral reader be deterred altogether from the in-vestigation of a subject which is not only extremelyimportant but in reality full of interest.
I invite the general student to notice, in the firstinstance, that the whole of the following line ofargument must be attentively followed. If a singleparagraph be omitted or slurred over, what followswill forthwith become perplexing. But I believe Ican promise him that, with this sole -proviso , he willmeet with no difficulties of an important nature. Onthe other hand, should the more advanced studentby chance peruse these pages, I invite him to con-sider that the account here presented is intendedonly as a sketch, and that if certain details are but