While rapid progress was being made in this and other directions, the wheels of industry were humming merrily at the Edison Tube Works, for over fifteen miles of tube conductors were required for the district, besides the boxes to connect the network at the street intersections, and the hundreds of junction boxes for taking the service conductors into each of the hundreds of buildings. In addition to the immense amount of money involved, this specialized industry required an enormous amount of experiment, as it called for the development of an entirely new art. But with Edison's inventive fertility--if ever there was a cross-fertilizer of mechanical ideas it is he--and with Mr. Kruesi's never-failing patience and perseverance applied to experiment and evolution, rapid progress was made. A franchise having been obtained from the city, the work of laying the underground conductors began in the late fall of 1881, and was pushed with almost frantic energy. It is not to be supposed, however, that the Edison tube system had then reached a finality of perfection in the eyes of its inventor. In his correspondence with Kruesi, as late as 1887, we find Edison bewailing the inadequacy of the insulation of the conductors under twelve hundred volts pressure, as for example: "Dear Kruesi,--There is nothing wrong with your present compound. It is splendid. The whole trouble is air-bubbles. The hotter it is poured the greater the amount of air-bubbles. At 212 it can be put on rods and there is no bubble. I have a man experimenting and testing all the time. Until I get at the proper method of pouring and getting rid of the air-bubbles, it will be waste of time to experiment with other asphalts. Resin oil distils off easily. It may answer, but paraffine or other similar substances must be put in to prevent brittleness, One thing is certain, and that is, everything must be poured in layers, not only the boxes, but the tubes. The tube itself should have a thin coating. The rope should also have a coating. The rods also. The whole lot, rods and rope, when ready for tube, should have another coat, and then be placed in tube and filled. This will do the business." Broad and large as a continent in his ideas, if ever there was a man of finical fussiness in attention to detail, it is Edison. A letter of seven pages of about the same date in 1887 expatiates on the vicious troubles caused by the air-bubble, and remarks with fine insight into the problems of insulation and the idea of layers of it: "Thus you have three separate coatings, and it is impossible an air-hole in one should match the other."