436 Mr. J. Blackwall's Characters of some 



them, are equal. Now, this condition continuing to be satis- 

 fied, let us suppose the third point of support to move into 



the same line with the other two. The fractions ^r express- 

 ing the evanescent ratio of each elementary triangle to the 

 whole triangle, will then manifestly have the value £. And 

 three points of support in the same straight line will each of 

 them sustain the same pressure. 



Prony has cited the case of a mass supported upon four 

 points, of which he asserts one to be superfluous, the whole pres- 

 sure being borne by three. Speaking of the same case, Euler 

 has the following observation : " Verum si pondus quatuor 

 pedibus piano insistat determinatio singularum pressionum, 

 non solum multo magis ardua deprehenditur, sed etiam pror- 

 sus incerta et lubrica videtur ; statim enim ac illi pedes non 

 cxactissime inter se faerint <zguale$, ita ut omnes piano pariter 

 innitantur manifestum est totnm pondus a ternis tantum pedi- 

 bus suste atari" &c. Thus it is manifest that Euler admitted 

 the necessity of each point of resistance sustaining a certain de- 

 finite portion of the whole pressure, provided all were simi- 

 lar/!/ situated in reference to the plane qf support. For he as- 

 serts the possibility of three out of the four points supporting 

 the whole mass, only as contingent upon an inequality in the 

 positions of those points in relation to the plane of support. 

 King's College, Oct. 14, 1833. 



LXX. Characters of some undescribcd Genera and Species of 

 Araneidaj. By John Blackwall, Esq. F.L.S. $c* 



Tribe, TubiteljeA La treille. 

 Genus, Clubiona, J 



Clubiona saxatilis. 



THIS species has the cephalothorax large, gloss}', convex above, depressed 

 and broadly truncated before; the sides, which are somewhat depress- 

 ed, are marked with slight furrows, extending from the^uperior part to the 

 lateral margins, and a narrow, elongated indentation occurs in the medial 

 line of the posterior region. Eyes disposed in two transverse, parallel rows 

 on the anterior part of the cephalothorax j the intermediate ones of the 

 anterior row, which is the shorter of the two and situated immediately 

 above the frontal margin, are the smallest of all, and the lateral eyes of 

 each row are seated on tubercles united at their bases. Mandibles very 

 powerful, vertical, triangular, exceedingly prominent at the base, provided 

 with two rows of teeth and a dense fringe of hair on the inner side. Max- 

 illae robust, curved towards the lip, enlarged at the base where the palpi 

 are inserted, and at the extremities, which are obliquely truncated on the 

 inner side and fringed with hair. Lip longer than broad, truncated at the 

 apex. Pectus heart-shaped, inclining to oval, glossy. Legs and palpi strong, 



* Communicated by the Author. 



