The Wealth of Nations by Part 1 Chapter 10 Page 16

therefore, commonly earn double and triple the wages of common labour, it ought not to seem unreasonable that coal-heavers should sometimes earn four and five times those wages. In the inquiry made into their condition a few years ago, it was found that at the rate at which they were then paid, they could earn from six to ten shillings a day. Six shillings are about four times the wages of common labour in London, and in every particular trade the lowest common earnings may always be considered as those of the far greater number. How extravagant soever those earnings may appear, if they were more than sufficient to compensate all the disagreeable circumstances of the business, there would soon be so great a number of competitors as, in a trade which has no exclusive privilege, would quickly reduce them to a lower rate.