PUN, an expression where a word has at once different meanings. The practice of punning is the refuge of those who wish to pass for wits, without having a grain of it in their composition. James I. of England delighted in punning; and the taste of the sovereign was studied by the courtiers, nay, even by the clergy. Hence the sermons of that age abound with this species of false wit. It continued to be more or less fashionable until the reign of Queen Anne, when Addison, Swift, Pope, and Arbuthnot, with the other real wits of that age, united their efforts to banish punning
There is one circumstance that we have not taken any notice of, viz. the gradual acceleration of the motion of water in pumps. When a force is applied to the piston, it does not in an instant communicate all the velocity which it acquires. It acts as gravity acts on heavy bodies; and if the resistances remained the same, it would produce, like gravity, an uniformly accelerated motion. But we have seen that the resistances, which are always measured by the force that just overcomes them, increase as the square of the velocity increases. They therefore quickly balance the action of the moving power, and the motion becomes uniform in a time so short that we commit no error of any consequence by supposing it uniform from the beginning. It would have prodigiously embarrassed our investigations to introduce this circumstance; and it is a matter of mere speculative curiosity; for most of our moving powers are unequal in their exertions, and these exertions are regulated by other laws. The pressure on a piston moved by a crank is as variable as its velocity, and in most cases is nearly in the inverse proportion of its velocity, as any mechanician will readily discover. The only case in which we could consider this matter with any degree of comprehensibility is that of a steam-engine, or of a piston which forces by means of a weight lying on it. In both, the velocity becomes uniform in a very small fraction of a second.
We have been very minute on this subject. For although it is the only view of a pump which is of any importance, it of course is hardly ever understood even by professed engineers; and this is not peculiar to hydraulics, but is seen in all the branches of practical mechanics. The elementary knowledge to be met with in such books as are generally perused by them, goes no farther than to state the forces which are in equilibrio by the intervention of a machine, or the proportion of the parts of a machine which will set two known forces in equilibrio. But when this equilibrium is destroyed by the superiority of one of the forces, the machine must move; and the only interesting question is, what will be the motion? Till this is answered with some precision, we have learned nothing of any importance. Few engineers are able to answer this question even in the simplest cases; and they cannot, from any confident science, say what will be the performance of an untried machine. They guess at it with a success proportioned to the multiplicity of their experience and their own sagacity. Yet this part of mechanics is as susceptible of accurate computation as the cases of equilibrium. We therefore thought it our duty to point out the manner of proceeding so circumstantially that every step should be plain and easy, and that conviction should always accompany our progress. This we think it has been in our power to do, by the very simple method of substituting a column of water acting by its weight in lieu of any natural power which we may chance to employ.
To such as wish to prosecute the study of this important part of hydraulics in its most abstruse parts, we recommend the perusal of the Dissertations of M. Pitot and M. Bossut, in the Memoirs of the Academy of Paris; the Dissertations of the Chevalier de la Borda, 1766 and 1767; and the Hydraulique of the Chevalier de Buat.
from polite composition. It is still admitted sparingly in conversation, and only from those who cannot furnish anything better.