Mr. J. R. Napier un Velocity of Ships. 351 



two bent tubes, the one having its orifice looking forward, and the other 

 its orifice looking aft, and their other extremities connected with a bulbed 

 glass tube containing a little mercury. 



The velocity of the ship is indicated by the height to which the mer- 

 cury rises, and, as when the vessel is at rest or moved vertically, the 

 pressures on both the exposed orifices are, and always continue equal, 

 neither the varying immersions nor boisterous seas can have any influ- 

 ence on the heights to which the mercury will rise. 



Fig. 1 shows a reduced view of the instruments I have used. The 

 bent pieces i! and e' are fastened to the side of the vessel well under 

 water, and, I believe, may be fixed at any part, though I have hitherto 

 had them placed about the middle, in the en^ne compartment of steamers. 

 The instrument itself, I had thought, might be placed in any situation 

 where it could be most conveniently seen,— as, in the captain's cabin, for 

 instance. I now find that it is most effective when placed below the 

 external water level, as when placed above this level the indications be- 

 come uncertain from the accumulation of air which then separates from 

 the water. The stop-cocks B and M, and the short tubes at c and N, are 

 for the purpose of allowing air to escape if such is suspected to be present ; 

 and the stop-cocks a and k for regulating the size of the orifice, so as to 

 prevent the oscillations of the mercury. The pieces e' and l' were con- 

 nected with the instrument at E and l by block tin and Indian rubber 

 tubing. A scale of tenths of an inch placed alongside the glass tube, 

 with its zero level with the mercury in the bulb, shows the heights to 

 which the mercury rises, when the vessels are propelled at different 

 speeds. 



I imagined that the velocity would be indicated by the usual formula 

 t; = n V h, h being the height of the mercury, and that when v is taken 

 in knots per hour, and h in inches, n would be a constant quantity, if 

 not for all ships and at all velocities, at least for the same shiji at all 

 velocities, and, if constant, its value would be nearly 5, found by reduc- 

 ing the formula v^ = 2 gk from feet per second to knots per hour, and 

 to h inches of mercury instead of feet of water. The results I have re- 

 corded, however, do not exactly corroborate this ; but the experiments 

 are perhaps too few, and some of them not taken with suflicient care, 

 as in the first experiments I was not suflSciently acquainted with the 

 working of the instrument to take the necessary precaution for freeing it 

 of air, as in these trials it was generally placed above the water-level. 

 In the Fiery Cross these objections are removed. 



