406 Capt. Thomson on the Velocity of Waves. 



the ship: — the length of the hne between the stern and the log 



will be the measure of the apparent velocity of the waves for 



the interval of time, to which the rate at which the ship is 



sailing being added, the sum is the true velocity of the waves. 



The velocity of the waves per hour may be found as follows : 



Let a represent the rate at which the ship is sailing ; b the 



number of feet between the ship's stern and the log; c the 



interval of time in seconds; v the apparent velocity of the 



wave in sea miles, and x its true velocity ; then 



30 6 , , 



- — = V, and V -Y a = X. 



51 c 



Or the rule may be given as follows: — 



Multiply the number of feet between the ship's stern and 

 the log by 30, and the number of seconds in the interval by 

 51 ; then divide the former product by the latter, the quo- 

 tient will be the apparent velocity of the waves, in sea miles, 

 to which add the rate at which the ship is sailing : the sum will 

 be the true velocity. 



Instead of the log, it will be better to have something at the 

 . end of the line that can be more readily seen *; and if the line 

 measure exactly 510 feet (that is, equal to 10 knots), the calcu- 

 lation of the apparent velocity of the waves is rendered still 

 more simple : for here we have only to divide 300 by the se- 

 conds in the interval ; the quotient will be the apparent velocity 

 of the waves. 



The following is a list of ten trials made by me in latitude 

 36° 20' S., and longitude 10° E. At the time of these experi- 

 ments the ship was sailing at the rate of 6\ knots per hour ; 

 there was a moderate breeze from the westward, but it had 

 been blowmg hard a short time before from the same quarter. 

 The length of line out was 510 feet. 



Mean of the intervals 1 3,05 



* A bladder would answer well, being light, and conspicuous on the 

 sea. 



Now, 



