Figure of the Earth. 149 



The method employed by the French philosophers, in operations- 

 of the same nature, at several points of the arc of the meridian 

 passing through France, and subsequently by Biot at Unst and 

 Leith, is entirely different ; and is the invention of Borda. He 

 suspended in front of an astronomical clock a sphere of platinum, 

 by means of a slender iron wire, whose length was about four 

 times that of the clock pendulum ; the wire was made of iron, in 

 consequence of the great tenacity of that metal, which would 

 pennit it to be drawn of great fineness without rendering the 

 wire liable to break by the weight of the ball : the coincidences 

 of this wire, with a cross or mark upon the lens of the clock 

 pendulum, were observed by means of a small telescope placed in 

 front. After the pendulum was brought to rest, its extreme 

 length from the point of suspension to the lower surface of the 

 spherical body was measured, while it remained suspended ; the 

 distance between the centres of oscillation and suspension, or the 

 effective length, was found by calculation, founded on well 

 known formulae, on the supposition that the wire was devoid of 

 weight ; and a correction finally applied for the weight of the 

 wire. In this method, each observation is entirely independent 

 of any other, and rests upon its own merits ; whilst in the 

 method, employed by Captains Kater and Sabine, in which the 

 relation only is determined, which the length of the seconds 

 pendulum, at the stations to which the pendulum of comparison 

 is carried, bears to the length at the station of the fundamental 

 experiment, — the correctness of the absolute length at those sta- 

 tions will depend upon the accuracy of the original determination. 

 But this need not be a disadvantage, even in determining the 

 absolute length at the several stations, because the fundamental 

 experiment may be frequently repeated, until perfect accuracy 

 may be considered as attained ; and certainly is none, in the 

 application of the results to the deduction of the figure of the 

 earth : because in such case it is the relation only, and not the 

 absolute length, which is the object of precise inquiry. 



The method of Borda has recently been altered by Biot, who 

 uses a pendulum of less length ; the apparatus may thus be more 

 securely and conveniently carried from place to place, enclosed 

 in a glass-case. In spite of this improvement, the method of 

 Kater is well entitled to the preference ; the first experiment 

 requires no greater care or precautions, and will occupy less time 

 than every separate determination by the method of Borda ; and 

 in every subsequent trial, the British method is very much 

 more speedy, is capable of more accurate and comparable 

 results, and is less dependent either upon external circumstances, 

 or upon the skill of the observer. 



Besides the method which wc have described, our author made 



