Prof. Perry, after referring to what might have been said of the great things physical science has done for humanity, plunged into his subject. The work to be done was vast, and the workers altogether out of proportion to the task. The methods of measurement of electricity are not generally understood. Perhaps when electricity is supplied to every house in the city at a certain price per horse power, and is used by private individuals for many different purposes, this ignorance will disappear. Electrical energy is obtained in various ways, but the generators get heated; and one great object of inventors is to obtain from machines as much as possible electrical energy of the energy in the first place supplied to such machine. The lecturer called particular attention to the difference between electricity and electrical energy, and attempted to drive home the fundamental conceptions of electrical science by the analogies derivable from hydraulics. A miller speaks not only of quantity of water, but also of head of water. The statement then of quantity of electricity is insufficient, except we know the electrical property analogous to head of water, and which is termed electrical potential. A small quantity of electricity of high potential is similar to a small quantity of water at high level. The analogies between water and electricity were collected in the form of a table shown on a wall sheet as follows: We Want to Use Water. We Want to Use Electricity. 1. Steam pump burns coal, 1. Generator burns zinc, or and lifts water to a higher uses mechanical power, and level. lifts electricity to a higher level or potential. 2. Energy available is 2. Energy available is amount of water lifted x amount of electricity x difference difference of level. of potential. 3. If we let all the water 3. If we let all the electricity flow away through channel flow through a wire from one to lower level without doing screw of our generator to the work, its energy is all other without doing work, all converted into heat because the electrical energy is of frictional resistance of converted into heat because of pipe or channel. resistance of wire. 4. If we let water work a 4. If we let our electricity hoist as well as flow through work a machine as well as channels, less water flows flow through wires, less flows than before, less power is than before, less power is wasted in friction. wasted through the resistance of the wire.