The reasons which Professor Johnston assigns for believing that the _present_ wheat-exporting powers of the United States have been exaggerated, may be passed over with very little comment. The Board of Trade returns leave no room for doubt as to the quantity that has actually reached this country, and it is therefore unnecessary for us to follow him through his hypothetical estimate of the exportable grain, grounded on what they _ought_ to have had to spare for us. We may remark, however, that the data on which his calculations proceed are far from satisfactory. He shows that all the _wheat_ produced in the United States, as given in the estimates of the Patent Office, is inadequate to afford the eight bushels which in England we reckon to be requisite for the annual supply of each inhabitant--the population of the Union being about twenty-one millions, and the produce of wheat one hundred and twenty-seven millions of bushels. He does not overlook altogether the fact that wheat is not in America, as it is with us, almost the sole cereal food of the people; and he admits that a considerable allowance must be made for the consumption of Indian corn instead of wheat. But how much?--That is the question. The compilers of the State Papers at Washington estimate that Indian corn, buckwheat, and other grain, form so large a proportion of the food of the people, that they require only _three_ bushels of wheat per head; and no doubt they have good grounds for this calculation. Professor Johnston, however, without indicating any reason whatever for his assumption, has set down the consumption of each individual at _five_ bushels per annum; and thus, by a stroke of his pen, he reduces the average exportable surplus of the Union to _only_ three millions of quarters.