vi INQUIRY AND INTERPRETATION 151 



Bradley pondered over his observations for two or 

 three years before he was able to interpret them. The 

 solution, when found, proved to be very simple, and 

 may be illustrated by a familiar example of a similar 

 effect. Suppose we are seated in a train or motor car, 

 and that rain is falling vertically, or nearly so. The 

 lines followed by the rain-drops are seen upright while 

 the carriage is at rest, but immediately it moves they 

 seem to be tilted forward at the top ; and the faster 

 we travel the greater is the deviation from the vertical. 

 If we did not know that the rain was falling in vertical 

 lines, we might think it was falling from points ahead 

 of us, but we know that the slant is only apparent, and 

 is due to the motion of the train or car. The effect is 

 produced by a combination of the velocities of the 

 carriage and the falling rain, and is precisely similar 

 to that observed by Bradley in the case of the stars. 

 The earth travels around the sun at a rate of nearly 

 nineteen miles a second, and the stream of light coming 

 to us from every star has a velocity of 186,000 miles a 

 second. Consider the earth to be the carriage of the 

 illustration and light- waves to be the rain, and ifc is 

 easy to understand that the combination of the two 

 velocities must cause every star to be seen slightly 

 ahead of its true position. 



As the velocity of light is about ten thousand times 

 that of the earth, the amount of the- deviation due to 

 aberration may be represented by two lines each nearly 

 three hundred yards long drawn from the observer's 

 eye toward a star and separated at the upper ends by 

 the length of an inch. The angle between these two 

 lines is equivalent to the " constant of aberration " 

 discovered by Bradley by observations in a coal-cellar 

 in 1726. Bradley afterwards became Astronomer Royal, 



