24 SCIENCE PROGRESS 



joining the centre of the earth to the sun. The earth, in turning 

 on its axis, causes the drift to be twice a day horizontal at points 

 on the earth's surface within certain limits of latitude. Thus, in 

 this latitude, the motion of a point relative to the sun is hori- 

 zontal and due east and west at 12 o'clock on June 21, day and 

 night. At other times of the day and year various angles are 

 made with this direction. These variations differ for different 

 latitudes, so that they must be calculated for each place at which 

 experiments on the drift of the ether are carried out. 



But is there any reason for supposing that the ether is at rest 

 as regards the sun ? Long-continued observations of the stars 

 point to the conclusion that the sun itself has proper motion 

 relative to these stars. There is evidence that it is moving with 

 a velocity, not very different in magnitude from the earth's 

 orbital velocity, towards the constellation Vega. If this be the 

 case, the motion of the earth relative to those stars the observa- 

 tions of which lead to the above conclusion is the resultant of 

 both velocities. The direction of this, also, will vary with latitude, 

 time of day, and time of year; and there is the further con- 

 sideration that the magnitude will depend largely on the time of 

 the year. Taking the sun's proper motion as being 15 miles per 

 second in the direction indicated, it will be found that twice a 

 year — in March and September — the earth's orbital motion and 

 the sun's proper motion have nearly the same direction ; being, 

 however, in agreement in the former case and opposed in the 

 latter. From this point of view, the most favourable time of the 

 year for experiments on the measurement of ether-drift is the 

 month of March, at one period of which a velocity of about 31 

 miles per second is reached. But it must be remembered that 

 this velocity is merely that of the earth relative to a particular 

 set of stars which may themselves be moving with reference to 

 the ether. Astronomy can only measure motion of bodies 

 visible through the most powerful telescope for the time being 

 in existence. Is it not possible, and, indeed, probable, that the 

 whole visible universe is moving with respect to the ether? 

 This is the question which the following experiments have 

 aimed at answering; and the magnitude of the astronomically 

 determined velocity has been merely a guide for fixing the 

 sensitiveness necessary in the particular apparatus used. 



How then, supposing the ether near the earth to be free, is 

 the velocity of its drift past the earth to be measured ? Ether 



