ON THE DETERMINATION OF GRAVITY AT SEA, 93 



Captain Backhouse, R.N., kindly arranged that observations of the 

 barometer carried by the Battleship Royal Sovereign, which was anchored 

 in the neighbourhood, should be taken simultaneously. The writer 

 expresses his thanks to Lieut.-CommanclerW. R. Priston, R.N., for taking 

 these readings at intervals of five minutes during the three hours required 

 for the work. 



The marine barometers were all of the Kew standard pattern, those 

 on the Plucky being specially provided by the Meteorological Ofl&ce for 

 this work. The aneroid was that made by the Cambridge Scientific Com- 

 pany for the writer's previous experiments on gravity at sea,^ but on 

 this occasion a new mounting was devised. It was placed on a swinging 

 table which was hung from a hook by rubber cords, a wooden rod sup- 

 porting a heavy weight being screwed to the centre of the table to give it 

 a longer period of swing ; this acted very well, and obviated most of the 

 small vibrations due to the engines. Chief Artificer-Engineer S. Dawson 

 was chiefly responsible for its introduction. 



On the first day of the trials it was evident that when the ship was on 

 an east course the mercury barometer stood relatively higher than the 

 barometer on the Royal Sovereign, and also than the aneroid reading, 

 but that the difference only became marked when the Plucky had nearly 

 completed her run of nine or ten minutes ; this is, of course, due to the 

 lag of the mercury barometer. The residts were fully in accord with 

 the existence of an effect due to B.-W. motion, but it was decided to 

 repeat the observations with a longer run. Unfortunately the Royal 

 Sovereign was not ofi Spithead during the second trial, but the stationary 

 barometer readings on the first day gave the writer confidence in his 

 interpretation of the readings on the moving ship. On the second day 

 Captain Smith managed to find a longer stretch of sheltered water, which 

 permitted a run of twelve to fifteen minutes in a true east or west 

 direction at a speed of 22 knots, which gave better results. 



On this day (September 1, 1919) the wind at the outset was from 

 the south and estimated at about 3 knots. This direction was very favour- 

 able, because the speeds through the air were the same whether going east 

 or west, and the disturbing influence was reduced to a minimum. Later 

 in the day, however, the wind blew from the S.W. at about 8 knots, 

 subsequently freshening to, say, 10 knots, when the observations became 

 less reliable. Meeting the breeze on a west course, and doing 22 knots, 

 the destroyer pitched a little, causing the aneroid to ' pump ' appreciably ; 

 this, as has been explained elsewhere {he. cit.), introduces a systematic 

 error into its readings, making them too low. 



The results of the day's observations are shown in the diagrams (fig. 4), 

 in which the times are indicated on the horizontal axis and the reduced 

 readings of the barometers in millibars on the vertical axis. The mercury 

 barometer readings were treated according to the method of the Meteoro- 

 logical Office, the aneroid constant being determined by comparison 

 with the mercury instrument in harbour. It was found that one division 

 = 0-214 mb., rather less than in 1914. Since the aneroid possesses no 

 absolute scale, its graph occupies an arbitrary position on the diagrams 



' B.A Report, Newcastle, 1916. 



