496 



Messrs. Stewart and Loewy on the true 



[June 17, 



own considerations the most reliable, for reasons which will appear 

 presently. 



Speaking generally of the subject of the temperature-correction, we 

 must admit that our experiments do not tend to remove the difficulties 

 that seem to surround it. Our experience goes to prove, what the Indian 

 officers, entrusted with the pendulum-experiments and their reduction, have 

 also suspected, that the thermometers fixed to a so-called dummy-bar 

 (in order to place them in conditions similar to the swinging pendulum) 

 do not give a true indication of the real temperature of the pendulum. 

 If this is the case, the differences found by ourselves between the result 

 of the lower and that of the higher range can easily be understood. 

 Indeed, during the progress of these experiments it has always appeared 

 to us that not only the fluctuations indicated by the thermometers are 

 greater in range than those to which the pendulum itself is subjected, but 

 that they are also more rapid, and that the heavy and substantial pen- 

 dulum cannot keep time in these changes with the light and delicate 

 thermometers which are not absolutely sealed up into the substance itself. 

 In our experiments, each of which lasted from one to two hours, a high 

 temperature was usually produced at the beginning, and we attempted to 

 maintain the heat as much as possible by keeping the pendulum-room 

 closely shut on all sides during the progress of the experiment. The 

 inrush of cold currents can, however, obviously not be wholly prevented, 

 and a steady, more or less considerable fall of the temperature is recorded 

 in each of the experiments beyond 70°. This fall affects, in our opinion, 

 chiefly the thermometers themselves, while probably the pendulum main- 

 tains its higher temperature much longer. Thus we are inclined to 

 think that the mean temperature of the pendulum, if it could by some 

 means be exactly ascertained, might perhaps appear considerably higher 

 than the mean of the thermometer-readings recorded ; and to this circum- 

 stance we ascribe it mainly that the high temperature-experiments give 

 too large a correction, for in these experiments a greater difference in 

 the number of vibrations corresponds to an apparently smaller difference 

 in temperature. 



The question will, we have reason to hope, find its best solution by the 

 labours of Colonel Walker and Captain Basevi in India, where these 

 gentlemen can avail themselves of a great natural range, which will free 

 the experiments from the doubts and difficulties met by ourselves ; but we 

 cannot conclude this part without reminding experimenters of the words 

 with which, nearly forty years ago, General Sabine concluded the account 

 of his own experiments, and which have gained new force by the short- 

 comings of our own investigations *. 



" It seems therefore desirable, for the sake of experiments, which are 

 becoming greatly multiplied, and which are daily increasing in accuracy, 

 that means should be devised of obtaining the rates of pendulums in 

 * Phil. Trans. 1830, p. 253. 



