Complete as was the concession made by Grey to local claims, it would, nevertheless, be a grave error to think that he left no space for the assertion of imperial authority. No doubt it was part of his system to reduce to a minimum the occasions on which interference should be necessary, but that such occasions might occur, and demand sudden and powerful action from Britain, he ever held. Even in matters of a character purely domestic, he believed, with Lord John Russell, that intervention might be necessary, and he desired to prevent danger, not by minimizing the powers of the imperial authority, but by exercising them with great discretion.[46] It was perhaps with this conservation of central power in view that {272} he was willing to transfer to the British treasury the responsibility of paying the salary of the governor-general, provided the colonists would take over some part of the expenses and difficulties of Canadian defence. But the extent to which he was prepared to exalt the supremacy is best illustrated in the control of imperial commerce. A great change had just been made in the economic system of Britain. Free trade was then to its adherents not an arguable position, but a kind of gospel; and men like Grey, who had something of the propagandist about them, were inclined to compel others to come in. Now, unfortunately for Canada, free trade appeared there first rather as foe than as friend. As has already been seen, the measures of 1846 overturned the arrangement made by Stanley in 1843, whereby a preference given to Canadian flour had stimulated a great activity in the milling and allied industries; and the removal of the restrictions imposed by the Navigation Acts did not take place till 1849. At the same time the United States, the natural market for Canadian products, showed little inclination to listen to talk of reciprocity; and the Canadians, seemingly deprived of pre-existing advantages by Peel's action, talked of retaliation as a means of {273} bettering their position, at least in relation to the United States. Grey, however, was an absolute believer in the magic powers of free trade. "When we rejected all considerations of what is called reciprocity," he wrote to Elgin, "and boldly got rid of our protective duties without inquiring whether other nations would meet us or not, the effect was immediately seen in the increase of our exports, and the prosperity of our manufactures."[47] Canada, then, in his opinion could retaliate most effectively, not by setting up a tariff against the United States, but by opening her ports more freely then before. He had a vision, comparable although in contrast, to that of believers in an imperial tariff, of an empire with its separate parts bound to each other by a general freedom of trade. Besides all this, he had a firm trust that the evils which other nations less free than Britain might for a time inflict on her trade by their prohibitions, would