when they see others laugh; the common cause contributing also in a great degree to produce this effect. The same thing is evident even in adults; and shews us one of the sources of the sympathetic affections.
To these things it is to be added, that the alternate motions of the chest follow the same degrees of mental emotion with more and more facility perpetually, so that at last children (who are likewise more exquisitively sensible and irritable than adults) laugh upon every trifling occasion.
By degrees they learn the power of suspending the actions both of laughing and crying, and associate this power with a variety of ideas, such as those of decency, respect, fear, and shame: the incidents and objects, which before occasioned emotion sufficient to produce laughter, now occasion little or none, from the transmutation of their associations: their new associated pleasures and pains are of a more sedate kind, and do not affect them so much by surprise; and, which is a principal cause in respect of individuals, their equals laugh less, and, by forming them to the same model with themselves, make the disposition to laughter decrease still faster. For whatever can be shewn to take place at all in human nature, must take place in a much higher degree than according to the original causes, from our great disposition to imitate one another, which has been already explained.
It confirms this account of laughter that it follows tickling, as noted above, i.e. a momentary pain and apprehension of pain, with an immediately succeeding removal of these, and their alternate recurrency; also that the softer sex, and all nervous persons, are much disposed both to laugh and cry profusely, and to pass quickly from one state to the other. And it may deserve to be inquired, how far the profuse, continued laughter and mirth on one hand, sorrow, hanging the lip, and crying, on the other, which occur in madness, agree with it.
As children learn the use of language, they learn to laugh at sentences or stories, by which sudden alarming emotions and expectations are raised in them, and again dissipated instantaneously. And as they learnt before by degrees to laugh at sudden unexpected noises, or motions, where there was no fear, or no distinguishable one, so it is, after some time, in respect of words. Children, and young persons, are diverted by every little jingle, pun, contrast, or coincidence, which is level to their capacities, even though the harshness and inconsistency, with which it first strikes the fancy, be so minute as scarce to be perceived. And this is the origin of that laughter which is excited by wit, humour, buffoonery, &c.
But this species of laughter abates also by degrees, as the other before considered did, and, in general, for the same causes; so that adults, and especially those that are judges of politeness and propriety, laugh only at such strokes of wit and humour, as
