It is by the process of comparison that concrete objects are shown to be large or small. The artist, in drawing a picture of a building, will insert in his sketch forms of men, horses, wagons or some object the size of which is familiar to the public, so that by comparison the beholder gains some idea of the height of the structure.
In the same way achievements in the abstract are shown to be great or little. A journey undertaken two centuries ago was adjudged long or short by comparison with other contemporary travels, and diplomatic triumphs of past days were rated according to other feats in the same line at that time. It is by measuring the great accomplishments of the present day with those of the past that we arrive at a true estimation of the tremendous strides that have been taken in the march of progress in the last few decades.