12 RESEARCHES ON IRRITABILITY OF PLANTS 



we require time-measurements of the order of one-hundredth 

 of a second. It will be shown that these determinations 

 can be carried out with great precision, by means of the 

 intermittent dots themselves, when the periodicity of ^their 

 recurrence is rendered perfectly constant. For such pur- 

 poses, then, we require a frequency of intermittent contacts 

 amounting to something like a hundred times per second. 



It was clearly impossible to make the heavy plate- 

 carrier oscillate with such a high frequency. There remained 

 only the theoretical alternative of causing the writing- 

 point to vibrate to and fro, at the required frequency, so 

 as to make the necessary intermittent contacts with the 

 surface of the recording-plate. 



The advantage of this intermit fence may be understood 

 from a concrete example. It will be remembered that the 

 writing-point, under the action of the responsive fall, 

 moves parallel to the surface of the recording-plate. If 

 now, by means of some mechanism, the writing-point be 

 made to vibrate to and fro, say, ten times each second, at 

 right angles to the plate, this will in no way affect the record 

 beyond the fact that instead of a continuous line a dotted 

 line will be traced. The record will not now labour under 

 the defects inseparable from the friction of continuous 

 contact. Instead of this, we shall have the vibrating writer 

 tapping a record which is practically free from friction. 

 For it will be understood that, as in our concrete example, a 

 recording-point which is vibrating ten times each second will 

 execute one entire to-and-fro movement in one-tenth of a 

 second. The duration of contact, at the extreme forward 

 end of the swing, will represent only a small fraction, say 

 one-fifth, of the entire period of one vibration. Hence 

 after each contact, lasting only one-fiftieth of a second, the 

 recording-point is absolutely free to take up the movement 

 impressed upon it by the moving leaf. In a record lasting 

 for one second the sum of the intermittent contacts will then 

 amount to one-fifth, and the period of entire freedom to 

 four-fifths of a second. We can thus see the theoretical 



