earth is moving through space with varying velocity, it gives to the rays of light a slanting direction. By calculating the speed of the earth, and ascertaining the exact slanting direction of the rays, the velocity of light may be ascertained. This Bradley did, and showed that it coincided almost with the result arrived at by Roemer. Various other means have been adapted to test the results arrived at by these two astronomers. Fizeau, in 1849, was able to measure the velocity of light by using, not planetary or stellar distances, but by simply using distances in the city of Paris; while Foucault, in 1860, devised a method of measuring the velocity of light in air or any other medium. The results arrived at by these men leave no doubt as to the exact speed of light, which may now be reckoned to have a velocity of 186,000 miles, or 300,000,000 metres per second. Notwithstanding this great speed at which light travels, the nearest stars are so far off that their light takes about 3-1/2 years to reach the earth, while scientists tell us that some of the most distant stars are so remote, that their light takes thousands of years to reach our earth, travelling at the rate of 186,000 miles per second. From considerations like these we get a dim conception of the almost illimitable extent of the universe. Now let us try to understand what this rate of motion really means. We have to remember that light is caused by wave motions in the Aether, so that we have here a wave motion which is travelling through the Aether at the enormous rate already quoted. Light takes about 8-1/2 minutes to travel from the sun to the earth, a distance of 92,000,000 miles. Our fastest trains do not travel 80 miles an hour, and if a train left the sun and continued its journey through space at that rate, it would take over 130 years before it reached our earth, while the light would perform the journey in 8-1/2 minutes. We have some idea of the velocity of a train travelling at 80 miles an hour; what, however, must be the velocity of a wave motion which travels 22,500 times as fast? In Art. 56 we have seen that all energy is the energy of motion, and therefore wherever we get motion of any kind or sort, there we must have energy accompanying it, or the power to do work. We have here, then, a source of energy in the aetherial waves known as light waves, with their enormous velocity which is almost inconceivable and illimitable. What must be the energy which exists in space due to the wave motion of the Aether? We have to remember on this point that we are no longer dealing with a frictionless medium, but that we are dealing with matter, only in a far more rarefied and far more elastic form than ordinary matter, but nevertheless matter just as air is considered matter, and, being matter, its very motion imparts to the light waves a power and a force which make them capable of doing work. The kind of work done will be considered later on, when