Egon S. Pearson 
29 
from considerations of time and as all the forms 
were not printed at the commencement this was 
not done. In some cases therefore a series was 
measured directly after it had been marked, and 
if the observer happened to remember that its 
estimates were considerably too large or too 
small, his judgment would almost certainly be 
influenced when marking the next succeeding 
series; the correlation of judgments within this 
second series would hardly be altered, but any 
natural secular change which had been occur- 
ring from series to series might be broken*. 
The measures of the observations were made 
with a ruler divided to fiftieth's of an inch, so 
that readings could be taken to one hundredth 
of an inch with fair accuracy. 
(b) Experiments C and D. 
These two experiments were carried out with 
the help of a chronograph. The instrument 
was run by clockwork, and had a paper tape on 
^ which records could be made independently by 
.tb two pens worked by small electromagnets. One 
^ pen was put in circuit with a second's pendulum, 
a platinum pointer at the end of which made 
contact at each swing through the vertical 
position by cutting through a bead of mercury, 
the other pen was connected with a tapping 
key. The rate of the driving clock was not 
quite uniform, and the pendulum second-marks 
on the tape were therefore necessary in reckon- 
ing the intervals between the marks made by 
the other pen, corresponding to taps of the key. 
As the estimate in both experiments was one 
of 10 seconds, it was found that except for a 
few cases in Experiment D-f, the true value of 
the time interval between the taps could be 
represented with sufficient accuracy bythe factor 
e/p, where, 
* See p. 49, remark in Table I, regarding Series IX 
and X. 
f In Experiment D, some of the estimates had values 
nearer 20 seconds than 10 seconds, and here half the dis- 
tance on the tape between the nearest corresponding 20 
seconds was taken for p. 
