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LX VII. On a proposed Barometric Pendulum, for the Registration 

 of the Mean Atmospheric Pressure during long Periods of Time. 

 By W. J. Macquorn Rankine, C.E., F.R.SS. Lond. and 

 Edinb. $c* 



Introductory Remarks. 



WHEN this paper was read, I was not aware that a baro- 

 metric pendulum formed part of an instrument sent 

 by Mr. G. F. Hall to the Great Exhibition of 1851. There is 

 this difference, however, between Mr. HalFs pendulum and that 

 here proposed, that the former oscillates in a plane, while the 

 latter is intended to revolve. It appears to me that the revolving 

 pendulum will possess a considerable advantage, iuasmuch as the 

 mercury will remain steady in the tube during each revolution, 

 instead of rising and falling with each vibration, as it would do 

 in the oscillating pendulum. 



It has been objected to the proposed barometric clock, that 

 its introduction would be superfluous, owing to the great accu- 

 racy which has been attained in the photographic method of 

 registration. With all due deference to the well-known skill 

 and experience of Mr. Welsh, with whom I understand that this 

 objection originated, I must decline to admit its relevancy ; for 

 I cannot assent to the principle, that the excellence of an exist- 

 ing mode of recording natural phenomena ought to form a bar 

 to the introduction of any new mode in other respects unobjec- 

 tionable. It appears to me, on the contrary, that much instruc- 

 tion is to be gained by recording the same phenomena by dif- 

 ferent processes. There is this important difference between the 

 photographic and the chronometric methods of ascertaining the 

 mean barometric pressure during long periods, — that an integra- 

 tion, which in the former process has to be made by measuring 

 and calculating the contents of a curvilinear figure described on 

 a series of photographic plates, is performed in the latter process 

 by the barometric clock itself. 



W. J. Macquorn Rankine. 



Glasgow, Oct. 27, 1853. 



The influence of temperature on the rate of clocks and watches 

 has naturally suggested to more than one person the idea of 

 employing the variations of the rate of a clock, regulated by a 

 pendulum of suitable expansibility, to register the mean tempe- 

 rature during long periods of time. 



I have now to propose another and a somewhat similar appli- 



* Communicated by the Author, having been read to the British Associa- 

 tion for the Advancement of Science, Section A, at Hull, September 10, 1853. 



