138 SUBMARINE CABLE LAYING AND EEPAIRING. 



The speed at which cable leaves the ship is calculated from, 

 the mean of the strophometer readings every half -hour, which 

 give the mean revolutions per minute of the drum. The 

 circumference of the drum in feet, plus the circumference of 

 the cable (expressed also in feet), gives the effective circum- 

 ference ; and this multiplied by the mean revolutions per 

 minute of the drum gives the speed of the cable in feet per 

 minute. Multiplying this by 60 gives feet per hour, and 

 dividing by 6,087 gives nautical miles per hour. 



Each type of cable differs in circumference, the large shore- 

 ends being about 8In., and the intermediate sizes correspond- 

 ingly less, down to about 2|^in. or Sin. for the lightest deep-sea 

 type. With this data the effective circumference on the drum 

 is worked out for each type of cable. The length of cable 

 payed out per hour is also checked by the revolution counter 

 on the drum shaft and by observing the time at which the 

 factory mile marks on the cable pass out of the ship. 



In order to ascertain the percentage slack the ship's position 

 at any time must be known. This is ascertained approximately 

 by dead-reckoning — that is, for a given course the distance 

 travelled by the ship from one position to another, as shown by 

 the log. The percentage slack is then found by taking the differ- 

 ence between the distance travelled and the length of cable payed 

 out, multiplying by 100 and dividing by the distance travelled. 

 By taking the log readings every half-hour the speed of the 

 ship through the water is determined. The log indicator is 

 generally fixed to the stern rail and operated by the rotation 

 of the line towing the small fan or propeller through the 

 water. Various forms of logs and the manner of using them 

 are described in Chapter I. 



The log gives with sufficient accuracy the number of miles 

 travelled by the ship through the water from one moment 

 of time to another. The measurement that is really required, 

 however, is the distance travelled over ground. The ship 

 follows the course set on the compass, and, if unaffected by 

 ocean currents or leeway, the distance on the log represents 

 correctly the distance covered over ground. But if the body of 

 the water in which the ship floats is itself in motion, relatively to 

 the sea bottom, or if weather beats the ship to leeward, the true 

 course over ground is at some angle with that of the compass. 



