Mr. J. Blackwall on some undescribed Araneidae. 18*7 



4>7T b 3 



It follows that m = — — = ja, and that 



o 



Mv* +(*v 2 = 2g (M-ft) (h-x). 

 rj vdv - M-ju, /■ 2/A . 



Hence ~iu ovf=g m^ = g v~ MJ ver y nearl y- 



and if V = the length of the seconds pendulum in air, I in 

 vacuum, 



f-f-iHf.-»-'(.-£> 



The coefficient by which the old correction is to be multi- 

 plied, is consequently 2. 



M. Bessel has obtained by experiment 1*956 for the value 

 of this coefficient. The experiments of Mr. Baily (Transac- 

 tions of Royal Society for 1832, p. 399*,) give 1*864 for spheres 

 \\ inch in diameter, and 1*748 for spheres 2 inches in dia- 

 meter, and consequently show that the coefficient is different 

 for spheres of different diameter. No such difference is re- 

 cognised either by the present theory, or by that of M. Poisson, 

 who has taken into account the effect of the friction of the 

 air against the surface of the ball. The theory of this differ- 

 ence, therefore, stands in need of further inquiry. Experi- 

 ment is the most proper means of determining the amount of 

 correction to be applied to the pendulum, but theory com- 

 bined with it may enable us to determine the causes to which 

 the correction is due. 



Papworth St. Everard, Aug. 2, 1833. 



XXXV. Characters of some undescribed Genera and Species 

 of Araneidae. By John Blackwall, Esq, F.L.S. fyc.\ 



Tribe, Inequitel^, Latreille. 

 Genus, Neriene. 

 Tj^YES eight in number, unequal in size, disposed in two 

 •*-**•' transverse rows on the anterior part of the cephalothorax; 

 the intermediate eyes of both rows form a trapezoid whose 

 anterior side is considerably the shortest ; the lateral ones are 

 placed obliquely, in pairs, each pair being seated on a small 

 eminence and geminated ; the posterior eyes of the trapezoid 

 are larger, and the anterior ones much smaller than the rest. 

 Maxillae strong, inclined towards the lip, slightly dilated at 



* An abstract of Mr. Baily's paper will be found in Lond. and Edinb. 

 Phil. Mag. vol. i. p. 379.— Edit. 



f Communicated by the Author ; by whom other undescribed Arancidce 

 were characterized in our last Number. 



2B2 



