An important consideration in relation to any proposal for reduction or increase of the letter rate, or, indeed, of any rate of postage, is, of course, the probable effect on the volume of traffic. Sir Rowland Hill, when he put forward his plan, laid stress on the increase in the number of letters which he anticipated would follow the adoption of his proposal. Since that time it has become almost an axiom that a reduction of rate will naturally and inevitably be followed by an increase of the traffic, more or less considerable, according as the reduction is large or small. Indeed, some writers have thought that the new postal system was based on a law of fixed relative proportions between a reduction of rate and the corresponding result on traffic. In point of fact, Sir Rowland Hill's estimates were based only partially on the probable effect of the reduction in stimulating traffic, and rather on the anticipation that, with a rate reasonably low, all that vast letter traffic which it was well known was being unlawfully dealt with outside the Post Office would be attracted to the lawful service. It is probable that a point of approximate satiety can be reached in the reduction of postage rates no less than in the reduction of the price of other commodities. A reduction would then result in only slightly increased consumption of the commodity--that is, in the case of letters, increase of the number posted. _Per contra_, a moderate increase of rate would result in a comparatively small reduction of the number of letters.[638] But moderate variations of postage on ordinary letters are difficult to make, since popular charges, such as a penny or halfpenny, while they offer obvious advantages from many points of view, are not susceptible of slight modifications.