100 



On the Indian Pendulum-observations. 



[Nov. 24, 



(2.) Captain Basevi's daily course of procedure is as follows. At 6 a.m. 

 he sets in motion the pendulum which is under observation. At 7 a.m. 

 he observes three coincidences and reads the thermometers and pressure- 

 gauge. Between 7 a.m. and 4 p.m. he observes a coincidence and reads 

 •the thermometers and the gauge, five times at intervals of 1^ hour. At 

 4 p.m. he closes this portion of the work by observing three coincidences 

 and again reading the thermometers and the gauge. Thus for nearly ten 

 hours of the day Captain Basevi never permits himself to be absent for more 

 than a few minutes at a time from the pendulums. These frequent ob- 

 servations are necessary in order that the temperatures may be exactly 

 determined. At 8 to 10 p.m. he observes transits. 



(3.) Originally it was expected that, by employing a vacuum-apparatus, 

 the pendulum might be vibrated for twenty-four hours before the vibrations 

 became too small for the observation of coincidences, and consequently that 

 the rate derived from the coincidences would be wholly independent of 

 irregularities in the clock's rate in different parts of the twenty-four hours. 

 But this would have necessitated observations of the temperature at regular 

 intervals throughout the twenty-four hours, which, as a rule, would have 

 been impossible, though a few such groups of observations have heen taken 

 experimentally. Moreover at the commencement of the operations the 

 vacuum-cylinder could not be made sufficiently air-tight to admit of so 

 protracted an observation. 



(4.) Each pendulum is observed a certain number of days with the 

 face to the front, and then as many days with the face to the rear. At the 

 first four stations observations were taken for five days on each face, making 

 altogether twenty days' observations for both pendulums ; as, however, it 

 was found that the theoretical probable error of the mean of the ten days' 

 observations by a single pendulum was only +"05 of a vibration, the 

 number of observations was subsequently limited to six days on both 

 faces, making altogether twelve days' work at each station. 



(5.) The observations are now being printed in the office of the Trigo- 

 nometrical Survey, and a few specimen pages accompany this note. A 

 preliminary abstract of the mean results by both pendulums is also given, 

 and a map indicating the positions of the stations of observation*. 



(6.) The results obtained hitherto are not final ; the coefficients of 

 the corrections for temperature and pressure have not yet been conclusively 

 determined, and the reductions to mean sea-level will probably be effected 

 when the calculations of the influence of local irregularities in the crust of 

 the earth have been carried to a greater distance from the stations than has 

 hitherto been practicable. 



(7.) Of these corrections the most important is that for temperature ; 

 the mean temperature of the observations ascends from a minimum of 54° 

 at the base station Kew, to a maximum of 88° at Namthabad, being a 



* [It has not been thought requisite to publish this map. — G. GK S.] 



