The activities of Edison have been of such great range, and his conquests in the domains of practical arts so extensive and varied, that it is somewhat difficult to estimate with any satisfactory degree of accuracy the money value of his inventions to the world of to-day, even after making due allowance for the work of other great inventors and the propulsive effect of large amounts of capital thrown into the enterprises which took root, wholly or in part, through the productions of his genius and energies. This difficulty will be apparent, for instance, when we consider his telegraph and telephone inventions. These were absorbed in enterprises already existing, and were the means of assisting their rapid growth and expansion, particularly the telephone industry. Again, in considering the fact that Edison was one of the first in the field to design and perfect a practical and operative electric railway, the main features of which are used in all electric roads of to-day, we are confronted with the problem as to what proportion of their colossal investment and earnings should be ascribed to him.