February 3, 192 1] 



NATURE 



733 



encountered were largely unknown, and certainly 

 untried, when the investigation was begun. 

 Whether Hacker's conclusion be confirmed or 

 rctuted, those who follow will not fail to benefit 

 by his early experiences. 



With a view to a further oceanographic gravity 

 survey, Hccker constructed apparatus based upon 

 :' different j^rinciple, one which had been unsuc- 

 -sfully tested at sea by William Siemens in 1859. 

 :)is time he used a barometer with a sealed cis- 

 I n, so that the column of mercury should be sup- 

 ported by the pressure of the contained air; the 

 .1 stern was immersed in a vacuum flask packed 

 with cork shavings to keep the temperature, and 

 therefore the pressure of the air, as steady as pos- 

 sible. Heckcr arranged that four of these instru- 

 ments should be mounted at the corners of a rect- 

 angular box containing apparatus for the photo- 

 i^raphic registration of the heights of the mer- 

 I ury columns. The box was mounted upon central 

 •imbals, designed to maintain the barometers 

 rtical as the ship rolled or pitched. It is clear 

 Mat if the pressure within the cistern is known 

 irom temperature observations, the value of g is 

 obtainable. 



I^'cturing in July, 191 3, Prof. A. F. H. Love 

 suggested that the voyage of the. British Associa- 

 tion to Australia in the following year might afford 

 a valuable opportunity for once more testing the 

 value of gravity at sea, and urged upon British 

 astronomers and physicists the importance of the 

 problem. His appeal proved irresistible. The 

 time for preparation for so great an undertaking 

 was very short; nevertheless, it was found pos- 

 sible, largely through the generous provision of 

 instruments by the Meteorological Office and the 

 Cambridge Scientific Instrument Co., to accumu- 

 late apparatus whereby tests could be carried out 

 hv three separate methods. Profs. Hecker and 

 i Hclmert were anxious to have the instrument 

 I already described tested at sea, and willingly 

 olTcred it for trial during the voyage ; it was, 

 therefore, brought from Strassburg in June, 1914, 

 and installed on s.s. Aicanius in Liverpool. 



Of the other pieces of apparatus one depended 

 upon much the same principle as that of Siemens 

 and Hccker — namely, the equilibrium of a column 

 of mercury supported by air pressure within a 

 closed vessel — but there were important differ- 

 ences ; it was arranged that the lower surface of 

 the mercury should always be brought to the same 

 level in the cistern, an adjustment determined by 

 I an electric contact with a point which completed 

 a telephone buzzer circuit. This ensured a con- 

 slant volume for the enclosed air. The height of 

 I Ik; barometer column was not directly observed, 

 lint estimated by measuring the length of the 

 tiircad of mercury, which it was necessary to intro- 

 '] duce into the apparatus through a fine capillary 

 fnhc in order to secure contact at the lower sur- 

 face. Assuming that the temperature remained 

 constant, a defect in gravity would demand the 

 addition of mercury in order to lengthen the baro- 

 i meter column sufBciently to balance the pressure 

 ■ of the air in the reservoir. The sensitivity could 



NO. 2675, VOT.. 106] 



be made great by adopting a high ratio between 

 the cross-section of the barometer tube and that 

 of the capillary index tube through which the 

 mercury was introduced. 



A valuable suggestion for the improvement of 

 this apparatus came from Sir Horace Darwin, 

 who pointed out that the bulk of the difficulty 

 occasioned by fluctuations of temperature might 

 be overcome by employing such a volume of mer- 

 cury within the apparatus that its expansion would 

 automatically provide the additional height of mer- , 

 cury in the barometer tube necessary to balance 

 the increase of pressure of the air in the reservoir 

 due to a rise in temperature. 



The third method consisted in comparing the 

 readings of a mercury barometer with those of an 

 aneroid ; both were open to the atmosphere, and, 

 as in the boiling-point method, we have the equa- 

 tion ^=/)^'/l to give us the variations of g, W 

 being now given by the aneroid, and h and p by 

 the barometer and its attached thermometer. The 

 aneroid was specially constructed and kindly lent 

 by Sir Horace Darwin for this research. On 

 board a ship the voyage of which carries it 

 through many degrees of latitude one of the 

 greatest difficulties is to obviate the effects of 

 large changes of temperature. On the outward 

 voyage this difficulty was very largely overcome 

 by the generous installation by Messrs. Alfred 

 Holt and Sons of a special refrigerating 

 chamber on s.s. Ascanius, which served as an 

 excellent laboratory. It was furnished with 

 a separate system of brine pipes, and though at 

 first there were rather large fluctuations, two 

 engineers on day and night duty eventually be- 

 came so adept at regulating the flow of brine that 

 during the latter stages of the voyage the varia- 

 tion amounted to considerably less than one degree 

 over a period of twenty-four hours, even though 

 the observer was frequently within the chamber 

 for an hour or more at a time. In future work it 

 should, if possible, be arranged that it is not 

 necessary for the experimenter to remain in the 

 chamber for lengthy periods, as it is detrimental to 

 health ; but the experience does suggest that it is 

 possible to overcome the very serious difficulty of 

 maintaining a large room at an approximately 

 steady temperature throughout the voyage. 



Unfortunately, the outbreak of war at the time 

 the British Association reached the shores of Aus- 

 tralia led to the ship being commandeered for 

 troops, so it was not possible to make use of this 

 laboratory on the return voyage, and a good deal 

 of the experience gained was wasted. A place 

 was, however, found in the refrigerating chamber 

 of the P. & O. R.M.S. Morca for all the appara- 

 tus, so the test was continued, though under very 

 unfavourable conditions. on the homeward journey. 



It had scarcely been hoped, when the 

 expedition was planned, to do more than obtain 

 experience and information which would serve as 

 a guide for future work upon gravity at sea, and 

 more than this is not claimed for the results. 

 Briefly, it may be stated that leaks which de- 

 veloped in Hccker's apparatus, and a troublesome 



