The dynamos belong to the alternating type, and are comparatively simple in construction. In a previous chapter upon the dynamo it was stated that the fundamental feature was the relation that the field-magnet and the armature sustained to each other, and that in some cases the field-magnet revolves while the part that is technically called the armature remains stationary. In other cases the armature revolves and the field-magnets are stationary. In the latter case brushes and commutators are used, to catch and transfer the generated electricity, while in the former these are not needed, which simplifies the construction of the machine. As we have stated, the dynamos used at Niagara are constructed with revolving field-magnets that are bolted on to the inner surface of the steel ring that is carried by the hood, so that there are no brushes connected with the machine except the small ones used to carry the current to the field-magnets. The current for power purposes is generated in a large stationary armature about ten feet in diameter and of the same depth as the revolving ring. The revolutions of the ring send out currents of alternating polarity, and each of the ten machines will furnish electrical energy equal to 5000 horse-power, so that when the work that is now under way is completed 50,000 horse-power can be furnished in the form of electricity. About 35,000 horse-power is now actually delivered to the various industrial enterprises. The dynamos are set horizontally, since the shaft which connects them with the turbine wheel stands in a perpendicular position. Not all of the energy that is developed by the water-wheel is converted into electricity, but some of it appears as heat. In order to prevent the heat from becoming so great as to be dangerous to the machine it must be constructed in such a way as to admit of sufficient ventilation for cooling purposes. The armature is so constructed that there are air-passages running all through it, and on top of the revolving hood are two bonnet-shaped air-tubes set in such a way as to force the air down through the armature, which carries off the heat and warms the power-house, on the principle of a hot-air furnace. This great machine--which, in a way, is so simple in its construction--when in action conveys to the mind of the beholder a sense of wonderful power. It is only when we stand in the presence of such exhibitions as may be seen in this power-house, devised and executed by the genius of man, and in that greater presence, the mighty Falls of Niagara, that we get something of a conception of the power of the silent yet potent energy of the great king of daylight, the sun. There are very many interesting details that work in connection with this great power-plant, some of which we will describe, in a general way.