207. If we wish to compare the value of any article at different periods of time, it is clear that neither any one substance, nor even the combination of all manufactured goods, can furnish us with an invariable unit by which to form our scale of estimation. Mr Malthus has proposed for this purpose to consider a day's labour of an agricultural labourer, as the unit to which all value should be referred. Thus, if we wish to compare the value of twenty yards of broad cloth in Saxony at the present time, with that of the same kind and quantity of cloth fabricated in England two centuries ago, we must find the number of days' labour the cloth would have purchased in England at the time mentioned, and compare it with the number of days' labour which the same quantity of cloth will now purchase in Saxony. Agricultural labour appears to have been selected, because it exists in all countries, and employs a large number of persons, and also because it requires a very small degree of previous instruction. It seems, in fact, to be merely the exertion of a man's physical force; and its value above that of a machine of equal power arises from its portability, and from the facility of directing its efforts to arbitrary and continually fluctuating purposes. It may perhaps be worthy of enquiry, whether a more constant average might not be deduced from combining with this species of labour those trades which require but a moderate exertion of skill and which likewise exist in all civilized countries, such as those of the blacksmith and carpenter, etc.(1*) In all such comparisons there is, however, another element, which, though not essentially necessary, will yet add much to our means of judging. It is an estimate of the quantity of that food on which the labourer usually subsists, which is necessary for his daily support, compared with the quantity which his daily wages will purchase. 208. The existence of a class of middlemen, between small producers and merchants, is frequently advantageous to both parties; and there are certain periods in the history of several manufactures which naturally call that class of traders into existence. There are also times when the advantage ceasing, the custom of employing them also terminates; the middlemen, especially when numerous, as they sometimes are in retail trades, enhancing the price without equivalent good. Thus, in the recent examination by the House of Commons into the state of the coal trade, it appears that five-sixths of the London public is supplied by a class of middlemen who are called in the trade Brass plate coal merchants: these consist principally of merchants' clerks, gentlemen's servants, and others, who have no wharfs of their own, but merely give their orders to some true coal merchant, who sends in the coals from his wharf: the brass plate coal merchants, of course, receiving a commission for his agency.