566 Professor Oliver Lodge [April 1, 



whether vertical or horizontal curvature. If you think it over you 

 will perceive that a wind will not prevent his doing it correctly ; the 

 line of hole will point to the shooter along the path of his bullet, 

 though it will not point along his line of aim. Also, if the shots are 

 fired from a moving ship, the line of hole in a stationary target will 

 point to the position the gun occupied at the instant the shot was 

 fired, though it may have moved since then. In neither of these 

 cases (moving medium and moving source) will there be any aberration 

 error. 



But if the target is in motion, on an armoured train for instance, 

 then the marker will be at fault. The hole will not point to the man 

 who fired the shot, but to an individual ahead of him. The source 

 wdll appear to be displaced in the direction of the observer's motion. 

 This is common aberration. It is the simplest thing in the world. 

 The easiest illustration of it is that when you run through a vertical 

 shower, you tilt your umbrella forward ; or, if you have not got one, 

 the drops hit you in the face ; more accurately, your face as you run 

 forward hits the drops. So the shower appears to come from a cloud 

 ahead of you, instead of from one overhead. 



We have thus three motions to consider, that of the source, of the 

 receiver, and of the medium ; and of these only motion of receiver 

 is able to cause an aberrational error in fixing the position of the 

 source. 



So far we have attented to the case of projectiles, with the object 

 of leading up to light. But light does not consist of j)rojectiles, it 

 consists of waves ; and with waves matters are a little different. 

 Waves crawl through a medium at their own definite pace ; they 

 cannot be flung forwards or sideways by a moving source ; they do 

 not move by reason of an initial momentum which they are gradually 

 expending, as shots do ; their motion is more analogous to that of a 

 bird or other self-j^ropelling animal than it is to that of a shot. The 

 motion of a wave in a moving medium may iSe likened to that of a 

 rowing boat on a river. It crawls forward with the water, and it 

 drifts with the water ; its resultant motion is compounded of the two, 

 but it has nothing to do with the motion of its source. A shot from a 

 passing steamer retains the motion of the steamer as well as that given 

 it by the powder. It is projected therefore in a slant direction. A 

 boat lowered from the side of a passing steamer, and rowing oif, re- 

 tains none of the motion of its source ; it is not projected, it is self- 

 propelled. That is like the case of a wave. 



The diagram illustrates the difference. Fig. 1 shows a moving 

 cannon or machine-gun, moving with the arrow, and firing a succession 

 of shots which share the motion of the cannon as well as their own, 

 and so travel slant. The shot fired from position 1 has reached A, 

 that fired from tlie position 2 has reached B, and that fired from posi- 

 tion 3 has reached C by the time the fourth shot is fired at D. The 

 line A B C D is a proloDgation of the axis of the gun ; it is the line of 

 aim, but it is not the line of fire ; all the shots are travelling aslant 



