86 ^fr. ^y. Gravatt on the Atlantic Cable. 



Upon tlic whole, I consider ffom a couiparison of experiments, 

 pvincii)ally those of Colonel Beaufoy, that when the velocity in 

 laiots per' hour is v, the resistance in pounds per fathom will, for 

 the cable here supposed, be nearly enough represented by •012i;'"^. 



For the sake of brevity I shall take only the example of this 

 cable at an angle of 20°, where the velocity is, we see, 4*8 knots 

 per hour. Now, taking the depth of the sea at 2000 fathoms, 

 we have L = D cosec a = 2000 x 2-92 = 5840 fathoms. 



Taking, for example, the slipping velocity of the cable equal 

 to the velocity of the ship, or paying out twice the necessary 

 length of cable, we find for a slipping velocity of 4-8 knots per 

 hour, a resistance of about -2 lb. per fathom, or the total slipping 

 resistance of the cable 1168 lbs., by which amount the strain on 

 the cable at the ship's stern will be reduced, or the greatest 

 strain will be 4000 — 1168 = 2832 lbs., or to reduce the strain 

 little more than one-fourth, we should at this velocity have to 

 waste half the cable. Again, a waste of 1 knot of cable per 

 hour, or a little more than 20 per cent, of the cable, would only 

 reduce the strain about 70 lbs. 



From the above, it appears that we are not able materially to 

 reduce the strain upon the cable without occasioning excessive 

 waste, unless at 'Leviathan' speed. 



Now if, on the other hand, we increased the tension of the 

 cable beyond that due to the simple depth of the sea, the cable 

 would form a curve concave to the horizon, and we might find 

 the form of this curve when the strain was 2D, 3D, or any mul- 

 tiple whatever of D ; but it would be such very bad engineering 

 to stretch or strain a cable over the bottom of the sea, when we 

 ought, on the other hand, really to allow waste (according to 

 circumstances) to sink into its irregularities, that although I 

 have considered this state of the cable, I think it practically 

 necessary to observe only, that if from any accident the paying 

 out of the cable is obliged to be discontinued, then, in order to 

 avoid a catena.rian strain that might be fatal, the motion of the 

 ship should instantly be reversed, and if in very deep water she 

 should run back until the cable hung vertically, or nearly so, a 



distance which is expressed by D — L vers a. or D( 1 -. ). 



' *' \ sm a / 



A cross current could only curve the free part of the cable 



sideways without adding to the strain, or occasioning waste, as 



wc readily see if we recur to the idea of the cable lying npon the 



supposed inclined ])lane surface; for although we made it curve 



in any nianiur by pressing against it sideways, it would, as before, 



be prevented from slipping down by the weight of a portion of 



the same cable, whose length was the perpendicular height of 



the plane. 



