ON THE DETERMINATION OP GRAVITY AT SEA. 95 



It was, however, chosen so that it is approximatvly just as much below 

 the mercury graph on the eastward run as it is above it on the west- 

 ward run ; the values of a vertical scale division are, of course, the same 

 for the two barometers. As the aneroid was moved a few inches on its 

 table at 12.10 p.m., the subsequent readings are not strictly comparable 

 with those which preceded that time. 



From the diagrams it indubitably appears that there is a rise of the 

 mercury barometer when the Destroyer steams east and a fall when she 

 steams west. The precise amount is difficult to estimate, but from the 

 first three observations, made when the wind was on the beam and the 

 sea smooth, the difference amounts to approximately 2 mbs. The sub- 

 sequent readings, though made imder less favourable conditions, in 

 general agree with this. Where the ' pumping ' was particularly notice- 

 able the diagrams are marked ' P,' and there it is that the aneroid is 

 erroneously low, and it is only at the end of the run, where the water was 

 smoother and the pumping less troublesome, and when enough time had 

 elapsed to enable the mercury to fall nearly to its true level, that the 

 aneroid reading exceeds the mercury reading. 



As a depression was approaching from the west, the fall in pressure 

 going west was greater than the rise going east. Fortunately the aneroid 

 possesses a very small lag, so with it small and rapid changes can be 

 detected which would escape a boiling-point method. Mr. F. T. Whipple 

 and the writer had tested the lag by taking the instrument up and down 

 in the lift at the Meteorological Office. Each aneroid reading is the 

 result of five separate observations, each of which took from two to 

 three seconds. 



The last two runs were made on north and south courses, and here, 

 save for one observation, the agreement is remarkably good, the difference 

 between the two barometers being in fact smaller than the probable 

 error of each measurement. 



The term involving the correction for E.-W. motion is 2 <dv cos A. sin a, 

 where w is the earth's angular velocity, v the speed of the ship, X the 

 latitude, and a the deviation of the ship's course from true north or 

 south. For V = 22 knots, X = 50° 46', and a = 90°, the expected 

 difference between east and west amounts to 2-15 mbs. From comparison 

 with the experimental results we may conclude with assurance that it is 

 essential that this term shall be introduced into all gravity determinations 

 made on a moving ship. 



In the Committee's Report for 1916 (Newcastle) the writer, basing 

 his conclusion upon an erroneous estimate of Hecker's exj^eriments, 

 assumed the necessity for including this term, and briefly discussed its 

 bearing upon meteorological phenomena. During the War the attention 

 of gunners was drawn to its application to ballistics. It is clear that a 

 shell fired east will weigh less than when fired west ; assuming a horizontal 

 velocity of 500 metres per second, a shell fired at an 8,000-yard range 

 will carry east about 80 yards further than if fired west in these latitudes; 

 At the Equator the difference amounts to 120 yards ; it seems useful, 

 therefore, to introduce a bearing correction into gunnery. The load which 

 an airship can carry also depends upon its speed and course ; flying east 

 at 60 knots an airship of 60 tons can carry about 100 pounds (the weight 



