92 REPORTS ON THE STATE OP SCIENCE. — 1919. 



were reliable ; if elbch had its own port-hole or its own cowl (as is 

 arranged in ' inside ' cabins) it is highly probable that there was a differ- 

 ence in the pressures in the cabins, and that the difference varied with 

 the speed of the ship and the direction of the wind. Experiments should 

 have been made to test their equality of pressure. 



It is, however, easy in the light of later knowledge to find fault with 

 Hecker's work, but after all it was the pioneer work in this branch of a 

 very difl&cult subject, and it has guided the work of all experimenters 

 who have followed. 



It was rather surprising to watch a barometer falling at the rate of 

 about 1 mb. a minute as the ship turned about ; it might be useful to 

 inform navigators of the effect of eddies in order that wrong meteorological 

 inferences may not be drawn from barometric observations on fast- 

 moving craft. 



III. The GtRavity Correction for the Ship's Motion in Longitude. 

 By Professor W. G. Duffield. 



Geophysicists who are interested in the determination of gravity at sea 

 will remember that, on the completion of his voyages over the Atlantic, 

 Indian and Pacific Oceans, Hecker published his conclusion that gravity 

 at sea conformed within narrow limits to the formula obtained by Helmert 

 from observations made at land stations. Eotvos, however, called his 

 attention to a possible source of error which had not been taken into 

 account. The ship when on an east or west course is subject to an 

 increase or a diminution of the centrifugal force acting upon her, which 

 results in an apparent decrease or increase in the value of gravity. During 

 the course of subsequent experiments in the Black Sea, Hecker made 

 two short series of observations to ascertain if this correction should, in 

 fact, be made, and came to the conclusion that it should. Reference 

 to Hecker's paper shows that he employed the boiling-point method, with 

 which it is very difficult to get consistent results, and that he had reasons 

 for rejecting the first set, which appeared to give positive results, and 

 also that in the second set, upon which he relies, there appears, to the 

 writer's judgment at least, to be a considerable degree of uncertainty. It 

 was partly for these reasons, and largely for reasons which are discussed by 

 the writer in his account of the influence of the motion of the ship through 

 the air, that it was considered of importance to make a special examination 

 of this point, in order that there should be no uncertainty in the matter ; 

 the effect of E.-W. motion is of such a magnitude that it might mask the 

 variations of gravity that it is required to examine at sea. 



Through the kindness of Captain Stapleton-Cotton, R.N., it was 

 arranged that the destroyer Plucky should steam east and west alter- 

 nately, while a comparison was made between the readings of the mercury 

 and aneroid barometers which had been installed on board. The captain 

 of the Plucky, Lieut. J. M. Smith, R.N., gave his whole-hearted co-opera- 

 tion, and to him and to the chief engineer is very largely due the successful 

 issue of the experiments. In order that we might have a check upon the 

 natural changes of pressure which occurred during the experiments, 



