Dr. Cornell, of Philadelphia, contributes to the November number of the Educator an article on sleep, from which we make the following brief extracts : No one who wishes to accomplish great thir.gs should deny himself the advantages of sleep or exercise. Any student will accomplish more, year by year, if he allows himself seven or eight hours to sleep, and three or four for meals and amusements, than if he labors at his books or with his pen ten or twelve hours a day. It is true that some few persons are able to perform much mental labor, and to study late at night and yet sleep well. Some require but little sleep. But such individuals are very rare. General Pichegru informed Sir Gilbert Blane that, during a whole year's eompaign, he did not sleep more than one hour in twenty-four. Sleep seemed to be at the command of Napoleon, as he could sleep and awake apparently at will. M. Guizot, minister of France under Louis Philippe, was a good sleeper. A late writer observes that his facility for going to sleep after rxtreme excitement and mental exertion was prodigious, and it was fortunate for him that he was so constituted, otherwise his health would materially have suffered. A minister in Trance ought not to be a nervous man ; it is fatal to him if he is. After the most boisterous and tumultuous sittings, at the Chamber, after being baited by the opposition in the most savage mannerthere is no milder expression for their excessive violencehe arrives home, throws himself upon a couch, and sinks immediately into a profound sleep, from which he is undisturbed till midnight, when proofs of the Moniteur are brought to him for inspection. The most frequent and immediate cause of insanity, and one of the most important to guard against, is the want of sleep. Indeed, so rarely do we see a recent case of insanity that is not preceded by want of sleep, that it is regarded as almost a sure precursor of mental derangement. Notwithstanding strong hereditary prdis-position, ill-health, loss of kindred or property, insanity rarely results, unless the exciting causes are such as to produce a loss of sleep. A mother loses her only child, the merchant his fortune, the politician, the scholar, the enthusiast, may have their minds powerfully excited ; yet if they sleep well, they will not become insane. No advice is so good, therefore, to those who have recovered from an attack, or to those who are in delicate health, as that of securing, by all means, sound, regular and refreshing sleep. To the discoverer of the law of gravitationSir Isaac Newtonwe also owe the first distinct philosophical elucidation of the great chemical law of affinities. "Sugar," said he, "dissolves in water, alkalies unite with acids, and metals dissolve in acids. Is not this an account of an attraction between their particles? Copper dissolved in aquafortis is thrown down by iron. Is not this because the particles of iron have a stronger attraction for the particles of the acid than those of copper ; and do not bodies attract each other with different degrees of force ?" THE MOST IMPORTANT RULE IN SHOOTING A RIFLE. In shooting a rifle, if you press the trigger gradually, so as not to know the precise second when the piece is to be discharged, you will avoid the nervous start which is the most common cause uf failure to hit the mark. OUR thanks are due to Mr. John C. Merriam, Corresponding Secretary of the American Engineers' Association, for the particulars kindly furnished us in relation to the experiments upon the expansion of steam by the Naval Commission.