262 Archdeacon Pratt on the Stability of Arches. 



received from Professor Erdmann of Leipzig a small quantity of 

 sulphate of oxide of thallium, and he gave me as much thereof 

 as I required for effecting the measurements, the results of which 

 were the following : — 



Th a 1 on the right . 

 Thai on the left . 



whence - 



126 15-5, 

 139 52-5, 



57=6° 48'-5, and\=0'"-000237088. 

 I obtained further for 



Th a 2 on the right . . . 146 46, 

 Th a 2 on the left .... 119 22; 

 and hence 



y = 13° 42', and \=0'"-000236838. 



As a mean, therefore, 



\=0'"-00023696, 

 or 



A,=O0005348 millim. 



Freiberg, April 25, 1863. 



XXXVII. On the Stability of Arches. 

 By Archdeacon J. H. Pratt. 



To the Editors of the Philosophical Magazine and Journal 



Gentlemen, 



HAVING been asked to give an opinion, on theoretical 

 grounds, regarding the stability of a bridge about to be 

 erected in this country, I calculated some formulae, which, as I 

 have not seen the subject treated in this way anywhere else, I 

 think may be of use to some of your readers. 



Fig. I. 



