Glow-lamps are made in all sizes--from tiny globes small enough to top a tie-pin to powerful lamps of 1000 candle-power. Their independence of atmospheric air renders them most convenient in places where other forms of illumination would be dangerous or impossible; _e.g._ in coal mines, and under water during diving operations. By their aid great improvements have been effected in the lighting of theatres, which require a quick switching on and off of light. They have also been used in connection with minute cameras to explore the recesses of the human body. In libraries they illuminate without injuring the books. In living rooms they do not foul the air or blacken the ceiling like oil or gas burners. The advantages of the “Edison lamp” are, in short, multitudinous. Cheapness of current to work them is, of course, a very important condition of their economy. In some small country villages the cottages are lit by electricity even in England, but these are generally within easy reach of water power. Mountainous districts, such as Norway and Switzerland, with their rushing streams and high water-falls, are peculiarly suited for electric lighting: the cost of which is mainly represented by the expense of the generating apparatus and the motive power. One of the greatest engineering undertakings in the world is connected with the manufacture of electric current. Niagara, the “Thunder of the Waters” as the Indians called it, has been harnessed to produce electrical energy, convertible at will into motion, heat, or light. The falls pass all the water overflowing from nearly 100,000 square miles of lakes, which in turn drain a far larger area of territory. Upwards of 10,000 cubic yards of water leap over the falls every second, and are hurled downwards for more than 200 feet, with an energy of eight or nine million horse-power! In 1886 a company determined to turn some of this huge force to account. They bought up land on the American bank, and cut a tunnel 6700 yards long, beginning a mile and a half above the falls, and terminating below them. Water drawn from the river thunders into the tunnel through a number of wheel pits, at the bottom of each of which is a water-turbine developing 5000 horse-power. The united force of the turbines is said to approximate 100,000 horse-power; and as if this were but a small thing, the same Company has obtained concessions to erect plant on the Canadian bank to double or treble the total power. So cheaply is current thus produced that the Company is in a position to supply it at rates which appear small compared with those that prevail in this country. A farthing will there purchase what would here cost from ninepence to a shilling. Under such conditions the electric lamp need fear no competitor. But in less favoured districts gas and petroleum are again holding up their heads.