178 EAETHQUAKES. 



Where g = 32-19 and h = 5-671. 



It ^ill be observed that these two formulae (the first of 

 which is known as Eussell's formula, and the second as 

 Airy's) are practically identical. 



The apparent difference is in the average value as- 

 signed to the constant. 



For large waves such as we have to deal with, it would 

 be necessary, if we were desirous of great accuracy, to 

 increase the value of h by some small fraction of itself. 

 We might also make allowance for the different values of 

 g, according to our position on the earth's surface. With 

 these formulae at our disposal it is an easy matter, after 

 having determined the velocity with which a wave was 

 propagated, to determine the average depth of the area 

 over which it was transmitted. 



In making certain earthquake investigations the re- 

 verse problem is sometimes useful — namely, determining 

 the velocity with which a sea wave has advanced upon a 

 shown line, from a knowledge of the depth of the water 

 in which it has been propagated. 



Calculations of the average depths of the Pacific, de- 

 pendent on the velocity with which earthquake waves 

 have been propagated, have been made by many investi- 

 gators. 



In most cases, however, in consequence of having 

 assumed the wave to have originated on a coast line, 

 when the evidence clearly showed it to have originated 

 some distance out at sea, the calculations which have 

 been made are open to criticism. The average depths 

 which I obtained for various lines across the Pacific 

 appear to be somewhat less than the average depth as 

 given by actual soundings. We must, however, remember 

 that the common error in actual soundings is that they 

 are usually too great, it being difficult in deep-sea sound- 



