Dr. E. H. Hall lectured last evening in the Jefferson Physical Laboratory lecture room on Prime Motors and their Sources of Power.

A prime motor, said the lecturer, is an instrument taking its energy from nature and applying it to the uses of man. All mechanical tasks are done by the overcoming of resistance; in fact the overcoming of resistance is the definition of work.

There are three principal sources of energy by which work may be done, motion, attraction and repulsions. The first is called kinatic energy and the last two are called potential energy.

Fuel is one of the most important sources of energy, and has been ever since the invention of the steam engine; but even now the methods of utilizing it are very imperfect. In the best engines only about one-fifth of the energy of the coal is made use of. Wind has long been used as a source of energy and its use is increasing especially on land. Owing to the inconstancy of the wind, if it is to be used much in the future, it will probably be in connection with the storage battery.

Rain as a source of energy is seen in waterfalls. It has been estimated that the force of Niagara would drive all the engines in the world. There is no reason why we could not transport the energy of Niagara in the form of electricity for motive power hundreds of miles away, if by some means the cost of conducting it could be removed.

One of the latest of Ericsson's inventions was his sun motor, by which the rays of the sun are utilized to heat a steam boiler. Although this motor is not now in use, still it must be regarded as one of the most important inventions of Ericsson's life.