An Experimental Study of Field Methods. 
548 
In Fig. 5, the x coordinates of the curves are the length of 
sights in meters, and the y ordinates are the recorded ampli¬ 
tudes of the vertical swing of crosswire. It will be seen that 
the greatest disturbance comes not in the hour of maximum tem¬ 
perature, but in the hour of maximum difference of temperature 
between air and earth. 
i 
m 
MIMi 
jf| 
m 
in 
Wm 
IS 
Ml f: 
PP 
Fig. 10.—Typical. Disturbance Curves Showing Effect of Hour of Day and Length 
OF MGHT ON THE DEGREE OF UNSTEADINESS. 
X=hour of observation. 
Y=product of number of vibrations by their amplitude. 
Thus in Figs 5, 6 and 7 this hour is 10—11 A. M. The curves 
of Fig. 8 may be said to be typical of sunny days during all ex¬ 
cept the cold winter months. During a cloudy day, but little 
unsteadiness is observed. The effect of different amounts of 
clouds is clearly shown by Fig. 9, which records a measurement 
of the unsteadiness at the same hour of three different days, but 
under same circumstances except the degree of cloudiness. It 
