Mr. Ivory on the Figure of the Earth. 343 



of the membrane suddenly ceases almost entirely ; but on the 

 fork being turned further, it immediately reappears. If we 

 distinguish in the prongs of the tuning-fork three different 

 surfaces; viz. 1. front and back surfaces; 2. side surfaces; 

 3. end surfaces ; we observe this disappearance of sound at 

 every transition from a front surface to a side one, and vice 

 verm: again, in every transition from a front surface to an 

 end one, and vice versa; but no cessation of sound is per- 

 ceived in the transition from a side surface to an end one, and 

 vice versa. These phaenomena are not distinctly audible when 

 there is any noise, for which reason the experiments will be 

 best made at night. 



The following differences in these phaenomena from those 

 mentioned in the treatise alluded, to (in the Jahrbuch 1826, 

 iii. p. 385) are remarkable: 1. That the membrane is a plane 

 extending far beyond the tuning-fork, and that every point of 

 it can be agitated with equal facility, whilst the reciprocating 

 inclosed column of air had only one narrow opening, and 

 could only resound when the undulations of sound penetrated 

 through this aperture ; 2. That the membrane did not reci- 

 procate, and nevertheless showed these phaenomena, whilst 

 with a column of air not reciprocating, at least if it be narrow, 

 nothing can be observed. 



LI V. On the Figure of the Earth, as deducedfrom Measurements 

 of different Portions of the Meridian. By J. Ivory, M.A f 

 F.R.S* 

 TJ AVING examined the figure of the earth as deduced from 

 •■--*• experiments with the pendulum, we are naturally led to 

 consider the same question as it depends upon the measure- 

 ments that have been made, of different portions of the meri- 

 dian. If this interesting inquiry be placed within the reach 

 of the human understanding, and be accessible to our industry, 

 the two views of it must be consistent ; and, by instituting a 

 comparison between them, we may both learn what is already 

 sufficiently well ascertained, and be directed in our future at- 

 tempts to the most important points that still remain uncer- 

 tain. 



Let a denote the equatorial radius of the earth ; e the ec- 

 centricity of the elliptical meridian ; <p an arc of the meridian 

 reckoned from the equator ; and X the latitude of the extre- 

 mity of $ : then 



(1— e*rinax)4 



* Communicated by the Author. 



If 



