An Experimental Study of Field Methods. 
545 
not here. But while engaged in this study of the air, it was 
noticed that rod readings, made when such disturbances were 
very marked, differed systematically from readings made under 
steady conditions of the air; a careful study was accordingly 
made to discover its cause. It was soon apparent that this 
cause was what the writer has called differential refraction. 
liiifill ##§§!#§ 
fig 
i k; 
I 
1 ■ 
— 
A: —Vs FpiT;!" :di' r 
ilm 11 11 f 111m if 1 If] 11 [Im H 
^BCEkilliHlillllll ftP Itn 
Is 
Fig. 13.—Typical Disturbance Curves Showing the Effect of Height of Target on 
Unsteadiness. Alsena Mountains, Ariz., 10 a. m., July 25, 1893. 
X=height of target. 
Y=product of number of vibrations by their amplitude. 
As used in this paper, the term “ differential refraction ” ex¬ 
presses the difference in the amounts which the two lines of 
sight, upper and lower, are refracted by the air. 
As the amount of this differential refraction has been foun d 
to vary directly with the unsteadiness of the image, some care¬ 
ful experiments were made to determine the relative amounts of 
the unsteadiness in strata from one-half to four meters above 
