Another way of procuring sleep mechanically was related to me by Mr. Brindley, the famous canal engineer, who was brought up to the business of a mill-wright; he told me, that he had more than once seen the experiment of a man extending himself across the large stone of a corn-mill, and that by gradually letting the stone whirl, the man fell asleep, before the stone had gained its full velocity, and he supposed would have died without pain by the continuance or increase of the motion.  In this case the centrifugal motion of the head and feet must accumulate the blood in both those extremities of the body, and thus compress the brain.

Lastly, we should mention the application of cold; which, when in a less degree, produces watchfulness by the pain it occasions, and the tremulous convulsions of the subcutaneous muscles; but when it is applied in great degree, is said to produce sleep.  To explain this effect it has been said, that as the vessels of the skin and extremities become first torpid by the want of the stimulus of heat, and as thence less blood is circulated through them, as appears from their paleness, a greater quantity of blood poured upon the brain produces sleep by its compression of that organ.  But I should rather imagine, that the sensorial power becomes exhausted by the convulsive actions in consequence of the pain of cold, and of the voluntary exercise previously used to prevent it, and that the sleep is only the beginning to die, as the suspension of voluntary power in lingering deaths precedes for many hours the extinction of the irritative motions.

21.  The following are the characteristic circumstances attending perfect sleep.

1.  The power of volition is totally suspended.

2.  The trains of ideas caused by sensation proceed with greater facility and vivacity; but become inconsistent with the usual order of nature.  The muscular motions caused by sensation continue; as those concerned in our evacuations during infancy, and afterwards in digestion, and in priapismus.

3.  The irritative muscular motions continue, as those concerned in the circulation, in secretion, in respiration.  But the irritative sensual motions, or ideas, are not excited; as the immediate organs of sense are not stimulated into action by external objects, which are excluded by the external organs of sense; which are not in sleep adapted to their reception by the power of volition, as in our waking hours.

4.  The associate motions continue; but their first link is not excited into action by volition, or by external stimuli.  In all respects, except those above mentioned, the three last sensorial powers are somewhat increased in energy during the suspension of volition, owing to the consequent accumulation of the spirit of animation.

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SECT.  XIX.

OF REVERIE.