in favor of old acquaintances, but we liked them better before. Digressions, perhaps, are cut out; some little rash speculation quietly withdrawn; some hit at an opponent suppressed; but they do not always command the same ready assent, or appear so interesting as they did in their old form.
These remarks do not apply to Prof. Tait. His lectures now before us, from their nature, belong to the class of composition for which we avow our predilection. They were delivered extempore to a scientific audience, and printed from short-hand notes. They lose nothing of their vigor, to use an expression of Lord Macaulay, by translation out of English into Johnsonese. We are allowed to seize the thought in the making, and, if it loses anything in grace, the loss is more than counterbalanced by power.
Those who wish thoroughly to understand the subject of this paper should study Prof. Tait's lectures on the souces of energy, and the transformation of one sort of energy into another. Matthew Arnold's phrase, "let the mind play freely round" any set of facts of which you may become possessed, often recurs to the mind on reading these papers. There is a rugged strength about Prof. Tait's extempore addresses, which taken together with their encyclopedic range, and the grim humor in which the professor delights, makes them very fascinating. They have another advantage. Men not professionally scientific find themselves constantly at a loss how to keep up with the rapid advance which has characterized recent years. One has hardly mastered a theory when it becomes obsolete. But in Prof. Tait we have a reporter of the very newest and freshest additions to scientific thought in England and on the Continent, with the additional advantage of annotations and explanations by one of the most trustworthy guides of our time.
We propose to discuss the books and papers whose titles are prefixed to this article, in so far as they throw fresh light on the probable length of time during which the solar system may be supposed to have existed. It is but in recent times that any materials have been amassed for forming an opinion on the subject. Before the end of the last century geology hardly existed as a science; an inquiry as to the age of the world would have been unhesitatingly answered by the assertion that the earth was created in six days, 4,004 years before the birth of Christ. Though further research has shown that the sacred text bears no such interpretation, those copies of the "Authorized Version of the Bible" which are enriched with notes and marginal references still keep up the formal assertion.
A story is told in Brydone's "Tour in Sicily" which will serve to recall the state of public opinion on the subject of chronology at the end of the last century. The Canonico Recupero, a Sicilian priest, was Brydone's guide when he explored Mount Etna. Recupero (who afterward wrote a history of his native mountain) told the traveler
