14 THE PRESIDENTIAL ADDRESS. 



observation before you are successful. It is the combination of actual 

 experience witli scientific knowledge that is essential. As the principle is 

 so fundamental, I may be allowed to illustrate it by an actual experience : — 



It was in the early years of the war that a body of young scientific 

 students from our Universities was assembled for the purpose of testing 

 on the battlefield the value of such methods of locating enemy guns as 

 were already known. In their mutual discussions and considerations it 

 became clear to them that the great desideratum was a method of 

 measuring very exactly the time of arrival of the air pulse, due to the dis- 

 charge of the gun, at various stations in their own lines. If the relative 

 positions of the stations were accurately known it would then become a 

 matter of calculation to find the gun position. But the pulse was very 

 feeble : how could it be registered ? Various methods were considered, 

 and among them was one which no doubt seemed far-fetched and unlikely 

 to be successful. A fine wire is made to carry an electric current by which 

 it is heated. If it is chilled, for example, by a puft' of cold air, the resistance 

 to the passage of the current increases, and this is an effect which can be 

 measured if it is large enough. If, then, the hot wire could be made to 

 register the arrival of the air pulse from the gim a solution of the problem 

 was in hand. No doubt this method occurred to several members of the 

 company ; it was certainly turned over in the mind of one of them who 

 had had considerable experience of these fine heated wires. They had 

 been in use about thirty years, having been employed for the measurement 

 of temperature in many circumstances where their peculiar characteristics 

 gave them the supremacy over thermometers of the ordinary form. But, 

 and this was the important point, was it to be expected that the effect, 

 though it must be there, would be big enough to see ? Could the faint 

 impulse from a gun miles away produce an obvious chill in a hot wire ? 

 On first thoughts it did not seem likely, and the suggestion lay in abeyance. 



But it happened that one summer morning an enemy aeroplane came 

 over at daybreak on a patrolKng expedition. The officer of whom 

 I have spoken lay awake in his bunk listening to the discharges of the 

 anti-aircraft guns and the more distant explosions of their shells. 

 Every now and then a faint whistling sound seemed to be connected with 

 the louder sounds. The wall of the hut was of felt ; it was in poor 

 condition and there were tiny rents close to his head as he lay. The 

 gun pulses made a feeble sound as they came through. This set the 

 officer thinking : if the pulse was strong enough to make a soimd, it might 

 be strong enough to chill a hot wire perceptibly. So the method was 



