Adam Smith enumerated the five following circumstances as accounting for the difference in money wages in different occupations: "First, the agreeableness or disagreeableness of the employments themselves; secondly, the easiness and cheapness, or the difficulty and expense of learning them; thirdly, the constancy or inconstancy of employment in them; fourthly, the small or great trust which must be reposed in those who exercise them; and, fifthly, the probability or improbability of success in them." In support of the first point he instances the fact that "the most detestable of all occupations, that of public executioner, is, in proportion to the quantity of work done, better paid than any common trade whatever." In the second place it is to be expected that in those occupations in which the labor supply is kept down either on account of the difficulty or the expense of learning the trade, other things being equal, wages will be higher than in occupations in which there are no restrictions upon those who desire to enter. Again, in certain trades in which there are seasons of unemployment, wages tend to be higher during the time of employment than in trades which are similar in other respects but in which the employment is constant. It would be a mistake, however, to suppose that the higher wages for the shorter period will, on the average, be sufficient to compensate for the time during which there is no employment. Smith illustrates the fourth point as follows: "The wages of goldsmiths and jewelers are everywhere superior to those of many other workmen, not only of equal, but of much superior ingenuity, on account of the precious metals with which they are intrusted." Finally, in those occupations where there is no likelihood of failure the supply of labor is likely to be greater than in those occupations where failure is likely, and thus those who succeed in the latter occupations are reasonably sure to achieve greater success.