20 



Pressure correction = ^ ^', where p is the pressure in millimetres of mercury at C. and t 



1 "T~ V " v/vOO I * 



the temperature inside the receiver. 



The " pressure correction " really depends on the density, and so on the temperature as well as the 



pressure of the air in the receiver ; that is how - ^ - is accounted for. 



1 + * 00367/ 



There should he an allowance made for the fact that the pressure p is due in a small degree to aqueous 

 vapour, whose density is only ;i of that of air under the same pressure. But to make this small allowance 

 one requires to know the vapour pressure, and of this we had no note. At low temperatures, however, 

 the vapour present is necessarily very small. 



Rak cmreclion = - - 00001 157RP, where R is the rate in seconds per day, P the period of the pendulum 

 in seconds. The rate correction is negative if R is positive or chronometer gaining. Sidereal time was 

 employed throughout. 



The correction for finite arc of oscillation is given in various forms. - That of the United States Coast 

 and Geodetic Survey is equivalent to 



- 0-01357 (n + ri)(n-n') p 



logio (/') 



where P is the period of the pendulum. 

 Also 



semi-arc in millimetres at start 



n = 



n = 



distance between scale and plane on which pendulum swings in millimetres ' 



semi-arc in millimetres at end 

 distance as above 



The value of one scale-division on the instrument was 3 millims., the distance between scale and mirrors 

 at Winter Quarters about 2858 millims. 



No experiments were made in the Antarctic for flexure coefficient. As it is a function of the pillar, no 

 certain allowance seems feasible. The types of pillar used were, however, sufficiently similar to make it 

 unlikely that the neglect of a flexure correction causes any serious error. 



The above formuUe assume C. and millim. as the standard temperature and pressure, as it is easiest 

 to reduce to these even when the mean temperature and pressure of the experiments are widely remote 

 from these values. 



THE MELBOURNE OBSERVATIONS (Komii'ier 10, 11, 1901). 



The pendulum apparatus was erected in the cellar of the Melbourne Observatory. Swings were taken 

 with two pendulums, Nos. 37 and 39. It was found impossible to stop the leakage of air into the receiver 

 when exhausted, and accordingly the observations were made at atmospheric pressure. The rates of the 

 chronometer 6711 employed in the experiments were determined by direct comparison with the sidereal 

 clock in the transit room of the Observatory. Observations with the two pendulums were also made by 

 Mr. P, BAHACCHI, Government Astronomer of Victoria. 



THE CHRISTCHUROU OBSERVATIONS, NEW ZEALAND. 



Sets of swings were taken at Christohurch Magnetic Observatory in November, 1901, before the sailing 

 of the "Discovery" for the Antarctic, and again on her return in May, 1904. The Observatory is in 

 latitude 43 31' 50" S., and longitude 172" 38' 9" E. ; it is situated on a large alluvial plain (Canterbury 

 Plain), and is 25 feet above the level of the sea. 



The observations were taken in the Absolute Magnetic House, a small wood building exposed to changes 

 of temperature ; therefore the temperature during the experiments had a largo range, and changed 

 rapidly, There was no way of obviating this difficulty, no cellar being available. 



