The laboratory note-books of this period--1878- 80, more particularly--show an immense amount of calculation by Edison and his chief mathematician, Mr. Upton, on conductors for the distribution of current over large areas, and then later in the district described. With the results of this canvass before them, the sizes of the main conductors to be laid throughout the streets of this entire territory were figured, block by block; and the results were then placed on the map. These data revealed the fact that the quantity of copper required for the main conductors would be exceedingly large and costly; and, if ever, Edison was somewhat dismayed. But as usual this apparently insurmountable difficulty only spurred him on to further effort. It was but a short time thereafter that he solved the knotty problem by an invention mentioned in a previous chapter. This is known as the "feeder and main" system, for which he signed the application for a patent on August 4, 1880. As this invention effected a saving of seven-eighths of the cost of the chief conductors in a straight multiple arc system, the mains for the first district were refigured, and enormous new maps were made, which became the final basis of actual installation, as they were subsequently enlarged by the addition of every proposed junction-box, bridge safety-catch box, and street-intersection box in the whole area.