This morbid condition of our intellectual faculties has been admirably described by Johnson, in his Rasselas. "To indulge the power of fiction, and send imagination out upon the wing, is often the sport of those who delight too much in silent speculation. He who has nothing external that can divert him, must find pleasure in his own thoughts, and must conceive himself what he is not; for who is pleased with what he is? He then expatiates in boundless futurity, and culls from all imaginary conditions that which for the present moment he would most desire; amuses his desires with impossible enjoyments, and confers upon his pride unattainable dominion. The mind dances from scene to scene, unites all pleasures, in all combinations, and riots in delights which nature and fortune, with all their bounty cannot bestow. In time, some particular train of ideas fixes the attention: all other intellectual gratifications are rejected; the mind, in weariness or leisure, returns constantly to the favourite conception, and feasts on the luscious falsehood whenever she is offended with the bitterness of truth. By degrees the reign of fancy is confirmed; she grows first imperious, and in time despotic. Then fictions begin to operate as realities, false opinions fasten upon the mind, and life passes in dreams of raptures or of anguish." The celebrated physician Boerhaave was once engaged in so profound a meditation that he did not close his eyes for six weeks. Any fixity of idea may be considered as a monomania. Pascal, being thrown down on a bridge, fancied ever after that he was standing on the brink of a terrific precipice, which appeared to him an abyss ever ready to ingulf him. So immutable was this dread, that when his friends conversed with him they were obliged to conceal this ideal peril with a chair, on which they seated themselves, to tranquillize his perturbed mind. This is an instance of a painful fixity of thought, the result of which is melancholic mania; whereas ecstatic exultation is the enjoyment of a delicious sensation unknown in our habitual earthly enjoyments, and beautifully expressed by Shakspeare, when Pericles thus addresses Helicamus-- O Helicanus! strike me, honoured sir; Give me a gash,--put me to present pain, Lest this great sea of joy, rushing upon me, O'erbear the shores of my mortality, And drown me with their sweetness. Archimides was heedless of the slaughter around him. Father Castel, the inventor of the ocular harpsichord, spent an entire night in one position, ruminating on a thought that struck him as he was retiring to rest. And it is related of an arduous student, that he was reflecting so deeply on some interesting and puzzling subject, that he did not perceive that his feet were burnt by the fire near which he was seated. VARIETIES OF MANKIND.