Page:Popular Astronomy - Airy - 1881.djvu/203

Rh the drops are large, then if you stand still for a moment, and observe the direction in which the drops are falling, when there is little or no wind, you will see that the drops fall vertically downwards; but if you walk forward, you will see the drops fall as if they were meeting you; and if you walk backward, you will immediately observe the drops of rain falling as if they were coming from behind you. This is an accurate illustration of the principle of the aberration of light, I will now offer another.

In Figure 52, let A be a gun in a battery, from which a shot is fired at a ship DE that is passing. Let ABC be the course of the shot. The shot enters the ship's side at B, and passes out at the other side at C. But in the meantime the ship has moved from the position DE to the position d e, and the part B where the shot entered has been carried to b. Now, if the ship were a sentient and reflecting being, when it perceived that the path which the shot made through it, entering at b and going out at C, was in the inclined direction b C, it would say, "The shot came from somewhere ahead." You will see in this, the effect of the combination of the movements; that the shot appears to have come from a part further ahead than it would have seemed to come from if the ship had been at rest. And you will also see that the inclination of the apparent direction of the shot b C to the true direction BC depends on the proportion of b B to BC, that is, on the proportion of the velocity of the ship to the velocity of the shot. The greater