1869.] 



Corrections to Pendulum Observations. 



489 



carefully conducted experiments, and also to supply those who are actually 

 engaged in pendulum-experiments at the present time with practically 

 valuable results, we proposed to ourselves to observe the behaviour of pen- 

 dulums of the different forms hitherto used in such researches in which 

 the pendulum is employed, at pressures varying through the whole range, 

 from the lowest obtainable in a receiver to the usual atmospheric pressure. 

 The carrying out of our intentions met, however, with many delays through 

 unavoidable circumstances, and there is, indeed, at present little prospect 

 of our being able to complete the whole of the original plan. We give, 

 therefore, here an account of some preliminary results which are, in our 

 opinion, not without practical importance, and which will certainly find 

 their use in the reduction of observations made with pendulums of a form 

 similar to that used by ourselves, viz. that form of reversible pendulum 

 known as " Kater's pendulum." 



4. The following is an account of the operations : — The pendulum was 

 swung in the Kew receiver, made of five pieces, two of metal and three of 

 glass, the parts fitting closely, and the whole being connected with siphon- 

 gauge and air-pump by tubes. One of the metal pieces is perforated be- 

 hind and in front, and the apertures are covered by plate glass for the 

 observation of the coincidences. 



The pendulum was swung at the following pressures : — 



I. At about \ of an inch 

 (lowest obtainable). 

 H. Between i and 2 inches. 



m. „ 2 „ 3 „ 

 iv. „ 3 „ 4 » 



X. At about 20 inches. 

 XL „ „ 25 „ 

 XII. At the full atmosphe- 

 ric pressure. 



V. Between 4 and 5 inches. 

 VI. „ 5 „ 6 „ 

 VII. „ 7 „ 8 „ 

 VIII. At about 10 inches. 

 IX. „ „ 15 „ 



At each pressure a good many observations were made, in order to ensure 

 reliable mean results. 



5. With reference to the registration of the observations, we have strictly 

 adhered to the method previously adopted after careful consideration, and 

 explained in our former account ; hence we need not here enter upon this 

 part again. Instead of registering one coincidence at the beginning, during 

 the progress, and at the end of an experiment, we have this time in most 

 cases observed three successive coincidences, and the arithmetical mean of 

 these, together with the mean of the corresponding registrations of arc, 

 temperature, and pressure, stands for one observation ; we think that this 

 method ensures greater correctness, although it is more laborious than that 

 previously adopted. 



6. The reduction of the observations comprises, as shown in the previous 

 paper alluded to above, several corrections to be applied to the number of 

 observed vibrations ; we shall mention here only those points which differ 

 numerically or experimentally from the numbers or methods explained in 

 that paper, which contains also an experiment with its full reductions. By 

 referring to these and the following remarks our method of procedure will 

 be so abundantly clear, we hope, that we shall be able to proceed immedi- 

 ately afterwards to the statement of the final results. 



2 o 2 



