VELOCITY OE WHITE AND OF COLOURED LIGHT. 
253 
to coincide in succession with the vertical wire. This done, a reading is taken, and 
the cylinder is turned round until another mark coincides with the vertical wire. So 
the operation is continued, first with the wheel marks, and afterwards with the 
clock marks. In reading off the marks on each side of a break in the traces care must 
be taken to notice whether a mark (or it may be even two marks) have been omitted. 
Care must also be taken to notice the number of whole revolutions of the cylinder. 
But it is only in the case of the clock-pen that this is necessary; and even in this case 
the omission of this precaution could hardly lead us into error. In this way we may 
obtain a series of readings for the wheel and clock respectively such as the following:— 
January 20, 1881, Ho. 6. 
Wheel. 
Clock. 
Reading. 
Differences. 
At mean reading. 
Reading. 
Alternate 
differences. 
Alternate second 
differences. 
231 
2,411 
1,437 
9,890 
22,557 
2,642 
23,072 
(22,419) 
329 
2,404 
5,046 
s, 32,309 
22,228 
295 
7,449 
45,300 
54,433 
22,124 
Signal at 3,810 
Signal at 24,108 
The signals are indicated by the commencement of the break in the pen traces. 
Notice that in the reading of the wheel marks the third one is a blank, indicating that 
one mark is omitted in the break. In consequence of this the corresponding 
“ difference ” is half of the difference between the two numbers 2,642 and 7,449. 
Notice also that in taking the clock differences we must subtract the alternate read¬ 
ings, partly because the clock cannot be perfectly “ in beat,” and partly because the 
electrical connexion is such as to make the length of the second, as registered, depend 
upon the direction of vibration of the pendulum. 
The “difference,” such as 2,411, in the column of the wheel record, is the number 
of divisions (or ten-thousandths of a revolution) passed over by the cylinder in the 
course of 100 revolutions of the toothed wheel. 
The “'alternate difference,” such as 22,419 in the column of the clock record, is the 
number of divisions passed over by the cylinder in the course of two seconds of time. 
If the cylinder rotated with perfect uniformity the quotient of these two numbers, 
viz., - 2 “A~r'=9 - 2986, would be the number of hundreds of revolutions completed by 
the toothed wheel in two seconds of time, and if we multiply this number by 50, we 
get 464‘93 revolutions per second as the speed of the toothed wheel. 
But as a matter of fact the chronograph-cylinder is constantly accelerated by the 
small weight, and retarded by the friction of the instrument. The total effect is 
generally a retardation, gradually diminishing the velocity of rotation of the chrono- 
