Black had a notable theory, entirely his own, about crossing the electric current.—Author, chap. i.
I have postponed the large subject of science, until I could enter upon it uninterruptedly, after I had taken my reader through our material and general progress, up to the dawn of the twenty-fifth century. With that century, as we have seen, comes so remarkable a change in the world's aspects and conditions, that it constitutes, as I have already said, a great era of division in my thousand years' retrospect. With that century, the world lost its distinctive nationality system, and finally completed its graduation into one great homogeneous society, to the immense advantage of all human progress, and, not least, of that science progress which I am now to record.
The science progress of these past thousand years may be divided into three grand eras, which are, respectively, that of the cross-electric discovery, the discovery of the duplication of the cross, and lastly
