113 



specting the figure of the earth. It having been shown by Clairaut 

 that the sum of the two fractions, expressing the ellipticity and the 

 diminution of gravity, from the pole to the equator, is always a 

 constant quantity, and equal to of the fraction, expressing the ratio 

 of centrifugal force, and that of gravity at the equator, it follows 

 that if the decrease of gravity from the pole to the equator be sub- 

 tracted from this constant quantity, the remaining fraction will ex- 

 press the ellipticity of the spheroid. The diminution of gravity may 

 be known by finding the difference of the length of two pendulums, 

 vibrating in equal times at the equator and pole, which are to each 

 other directly as gravitation ; but as such experiments cannot be 

 made at the pole, Captain Kater proceeds to describe the means of 

 obtaining the desired result by observations at intermediate stations ; 

 whence it appears that the length of the seconds pendulum at the 

 equator, deduced from the observations at Unst and Dunnose, is 

 39 - 00527 inches, and gravitation at the equator 16'040 feet; hence 

 the centrifugal force at the equator is -j-fc-g- of gravitation, or T i T of 

 gravity, which last being multiplied by - gives '0086505 for the 

 sum of the fractions, expressing the ellipticity of the earth and dimi- 

 nution of gravity from the pole to the equator. 



It appears from this result that, excepting the allowance for 

 height above the sea's level, the error in the number of vibrations of 

 the seconds pendulum at any particular station does not amount to 

 iVth of a vibration, which is about equal to 400,000th part of the 

 length, consequently gravitation may be determined to this degree of 

 accuracy by the apparatus employed; and in passing through a 

 country composed of materials of various degrees of density, the 

 pendulum may be expected to indicate such variations with much 

 precision. Irregularities that are observed in the decrease between 

 given latitudes, from the pole to the equator, are referable to this 

 irregular attraction ; thus the sudden increase of gravitation at 

 Arbury Hill is referred by Captain Kater to the granite of Mount 

 Sorrel in Leicestershire. 



Though the details of M. Biot's experiments are not yet published, 

 the author observes that it affords him no small gratification to learn 

 that the acceleration of the pendulum between London and Unst, 

 computed by that mathematician from his observations between Unst 

 and Formentara, differs only 0"'6 from the result of his own expe- 

 riments, a difference referable perhaps to the superior density of 

 Unst compared with that of the substrata of London. 



The Croonian Lecture. A further Investigation of the Component 

 Parts of the Blood. By Sir Everard Home, Bart. V.P.R.S. Read 

 November 4, 1819. [Phil. Trans. 1820, p. 1.] 



In this communication Sir Everard announces the existence in 

 certain animal structures of globules smaller than and inde- 

 pendent of those ordinarily belonging to the blood ; they were first 

 remarked by Mr. Bauer, during the microscopic examination of the 



