THE CABLE SHIP ON REPAIRS. 22& 



total number of revolutions made by the winding-in drum. One 

 form of rotometer, contained in a brass box about Hin. square 

 by Gin. long, consists of a set of discs, each with ten figures on 

 the rim, which show themselves in succession through holes in 

 the cover of the instrument as the impulses are given. By a 

 simple gear between the discs they are made to indicate units, 

 tens, hundreds, thousands, and tens of thousands of revolutions. 

 The rotometer box is fixed in any convenient position for reading 

 as at S, supported by a bracket, and is connected by light spur 

 gear to the drum shaft. The spur wheel W being of equal size 

 to that on the shaft into which it gears, the rotometer follows 

 the revolutions of the shaft and drum at the same rate. In 

 paying out rope or cable, if it is found necessary to stop and 

 pick up, the rotometer runs backwards with the shaft and 

 deducts the amount picked up, so that no calculation is required 

 for changes of direction. A note is taken of the figures as they 

 stand before the cable-end first winds round the drum ; and the 

 difference between this number and that observed at any subse- 

 quent period is the number of revolutions made. 



In calculating the length of cable passed over the drum from 

 the number of revolutions recorded, the circumference of the 

 cable has to be taken into account. For, considering one com- 

 plete turn of cable round the drum, it is evident that the edge 

 touching the drum all round is in compression or shorter than 

 its normal length, while the outside edge of the cable is in 

 extension or longer than its normal length. The correct length 

 of cable per turn is then in the path made by the centre of the 

 cable round the drum, this being in a neutral state. To find 

 the length of this circumference we must add the diameter of 

 the cable to that of the drum, and multiply by 3-1416, or, what 

 is the same thing, add together the circumferences of the drum 

 and cable (both expressed in feet); the result, divided by 6,087, 

 gives the length in nauts of cable picked up, and is entered in 

 the electrician's log in nauts and fractions of a naut to three 

 places of decimals. 



For example, suppose the rotometer showed 2,512 revolutions 



after picking up a cable of Sin. circumference, on a drum of 



18ft. in circumference. The length of cable picked up would 



, 18^x2,512 „ .„, 



be — ^ '= /,531 nauts. 



6,087 



