34 Mr Sang on the Proper Porm 



chain of structures, which runs through the series of organized 

 bodies. 



EXPLANATION OF THE FIGURES (PLATE I). 



Fig. 1. Pclonaia corrugata, a oral orifice of the test ; h anal orifice. 



2. Pelouaia glabra, a oral orifice ; b anal orifice. 



3. Anatomy of P. glabra, a, a bristle inserted into the respiratory 



sac through the oral orifice ; b, a bristle inserted in the man- 

 tle cavity through the anal orifice of the test ; c ganglion with 

 the nerves proceeding from it;d the shelf or transverse ridge 

 in the interior of the test and mantle ; e branchial vein inclosed 

 in a serpentine band, as in some of the other Ascidice ; f 

 branchial artery ; g generative organ of the left side, with a 

 bristle inserted into its duct ; 7i the stomach ; i anus ; kk cut 

 edge of the test. 



On the Proper Form for a Convertible Pendulum, By Edward 

 Sang, Esq. Actuary, Edinburgh, M.S. A. Communicated 

 by the Society of Arts for Scotland.* 



The determination of the exact length of a second's pendu- 

 lum is a problem of great importance ; as it serves, on the one 

 hand, to exhibit the intensity of gravitation at different places 

 on the surface of the earth, and, on the other hand, to verify 

 or to restore standards of linear measure. 



As any pendulum w^ith which we can operate is, of necessity, 

 compound, the computation of the length of a simple pendu- 

 lum must be founded on the dimensions of the various parts 

 of the pendulum actually used. The first attempts to deter- 

 mine tbe intensity of gravitation by this method, were accord- 

 ingly made by help of pendulums of as simple a structure as 

 possible, such as of a heavy sphere suspended by a slender 

 wire. But these attempts have given place to a method, ex- 

 ceedingly simple in itself and involving an elegant property 

 of oscillating bodies. The property is this, that if a second 

 knife edge be fixed upon a pendulum at a distance below the 

 first equal to the mean distance of oscillation, the motion on 

 the one-knife edge will be performed in the same time as on 



* Bead before the Society of Arts for Scotland, 12th April 1841. 



