432 Pendulum Base Observations for India. [1865. 



prevented such a course, and we availed ourselves of the elaborate series of 

 experiments on the temperature corrections of pendulums, made by General 

 Sabine (vide Phil. Trans. 1830, p. 251), which gives 0'44 vibration per 

 diem for each degree of Fahrenheit's scale. General Sabine found in a 

 former series this correction nearer to 0'43 ; and he says, in the above men- 

 tioned publication, " The probable error which may be incurred by employ- 

 ing the correction 0'44 for each degree as now determined, can only be very 

 inconsiderable ; but when the differences of temperature amount to 50, which 

 is a case of actual experience in pendulum observations, the question of 

 whether 0'43 or 0'44, for example, be the more correct value, involves an 

 uncertainty in the ultimate result of no less than half a vibration a day." 



The pendulums which we used were not those employed by General Sabine 

 in his determinations, but they were made by the same maker at the same 

 time, and very probably from the same kind of brass, and there cannot be 

 the least doubt that the true correction will lie between 0-43 and 0'44. 

 We have therefore adopted 0'435 for our reductions ; and as the greatest 

 difference in temperature between a single experiment and the mean is less 

 than 11, the greatest error would in this case amount only to y^-yths of a 

 vibration per diem, an error too small to affect seriously the mean result of 

 the whole. 



At the same time we must state that, as Colonel Walker and Captain 

 Basevi inform us, experiments will be made in India with both pendulums, 

 to ascertain their exact constants with regard to expansion, and that our 

 results will of course have then to be modified accordingly. 



