I well believe that a person naturally robust, with a clear and powerful brain, could bear twelve or fourteen hours' work every day for years together so far as the work itself is concerned, if only so large an expenditure of time left a sufficient margin for amusement, and exercise, and sleep. But the privation of exercise, by weakening the digestive and assimilative powers, reduces the flow of healthy and rich blood to the brain--the brain requires an enormous quantity of blood, especially when the cerebral matter is rapidly destroyed by intellectual labor--and usually brings on nervousness, the peculiar affliction of the over-driven mental laborer. This nervousness is Nature's kindly warning, preserving us, if we attend to it in time, from much more serious consequences. The best preventive of it, and often the only cure, is plenty of moderate exercise. The customs of the upper classes in England happily provide this in the best shape, that of amusement enjoyed in society, but our middle classes in large towns do not get nearly enough of it, and the most studious are always strongly tempted to neglect it altogether. Men of great imaginative power are commonly addicted to a habit which is peculiarly dangerous. They work as race-horses work, with the utmost intensity of effort during short spaces of time, taxing all their powers whilst the brilliant effort lasts. When Beckford wrote the wonderful tale "Vathek" in his twentieth year, he did it at a single sitting, which lasted for three days and two nights, and it cost him a serious illness. Several of the best poems by Byron were written, if not quite with equal rapidity, still on the same principle of composition at white heat. In cases of this kind, Nature provides her own remedy in the indolence of the imaginative temperament, which leaves large spaces of time for the action of the recuperative processes. The same law governs the physical energies of the carnivora, which maintain, or recover, their capacity for extraordinary effort by intervals of absolute repose. In its long spaces of mental rest the imaginative temperament recruits itself by amusement, which in England usually includes physical exercise of some kind.