NAVIGATION. 43 



at sea the " Dutchman's log " is a very primitive 

 method of measuring speed, in which a bottle is 

 thrown overboard from the bow, and its times of 

 passing two fixed marks, at a measured distance 

 apart on the ship, are observed. But primitive 

 as it is, it is more accurate than any other method 

 which has ever been practised for low speeds 

 and large ships. Suppose, for example, the marks 

 to be 250 feet apart, and the times of the floater 

 passing them to be 



ih. 1 7m. I2s. 

 and ih. I7m. 48^5. 



The interval, therefore, was 36^ seconds. Hence 

 the ship went 250 feet in 36 J seconds, and therefore 

 was going at the rate of 1000 feet in 146 seconds. 

 To find the rate in miles per hour, multiply the 

 number of feet per second by 3600 and divide by 

 6080. The result is 4-05. Therefore the ship was 

 going at the rate of 4*05 miles per hour. This 

 process would of course, be too troublesome for 

 ordinary use, requiring as it does two accurate 

 observers with watches having seconds hands, 

 and an assistant. It would be found, however, 



