385 



President, and were expressly manufactured for the purpose by Mr. 

 Mitchell, of Caleneck in Cornwall. In order to prevent the re- 

 duction of any portion of the lead entering into the composition of 

 the glass, a current of fresh air was introduced by a tube, and made 

 to pass along the surface of the fused glass. A very minute and cir- 

 cumstantial account is given of all the manipulations necessary for 

 conducting these processes in all their stages ; in some of which, 

 however, the best methods of proceeding still remain to be ascer- 

 tained, variations having been made up to the very last experiment, 

 and it is only by still more extensive experience that the author ex- 

 pects the proper arrangements will ultimately be settled. Directions 

 are given as to the occasional inspection of the glass during the pro- 

 cess, the mode of stirring by a rake of platina, and the plan devised 

 by the author of accelerating the disengagement and escape of bub- 

 bles, by throwing into the melted materials a quantity of pulverized 

 platina, mixed with fragments of the same kind of glass. The glass 

 which has been obtained by the mixture of materials above men- 

 tioned, constituting silicated borate of lead, has a specific gravity of 

 5 '44, and high refractive and dispersive powers, and perhaps also very 

 considerable reflecting power. It is softer than ordinary glass, but 

 less liable to be tarnished by sulphureous vapours, as they commonly 

 exist in the atmosphere ; and also less acted upon by moisture than 

 glass, in which potash enters as an ingredient ; it is also a much more 

 perfect electric than ordinary glass. An Appendix is subjoined, con- 

 taining descriptions of the rough glass furnace, and the finishing fur- 

 nace ; and also directions for preparing the spongy platina employed 

 by the author in the latter stage of the process, in order to promote 

 the disengagement of bubbles. 



Account of Levellings carried across the Isthmus of Panama, to ascer- 

 tain the relative Height of the Pacific Ocean at Panama, and of the 

 Atlantic at the mouth of the River Chagres ; accompanied by Geo- 

 graphical and Topographical Notices of the Isthmus. By John 

 Augustus Lloyd, Esq. Communicated by Captain Sabine, Sec.R.S. 

 Read November 26, 1829. [Phil. Trans. 1830, p. 59.] 



The author having received from General Bolivar a special com- 

 mission to survey the Isthmus of Panama, with the view of ascer- 

 taining the most eligible line of communication between the two 

 seas, arrived at Panama in March' 1828. Here he was joined by 

 Captain Falmarc, a Swedish officer of Engineers, in the Colombian 

 service. Anxious to lose no time in the prosecution of their objects, 

 they proceeded on the 5th of May to commence their operations, 

 resolving not to be deterred by the difficulties likely to arise from 

 the rainy season, which had just set in, from personal privations, and 

 even from the dangers to which they might expose their health. 

 Their line of survey commenced at Panama, and was continued along 

 the old road to Porto Velo till it came to the bed of the Chagres, a 

 river which falls into the Gulf of Mexico. The greatest height 



VOL. II. 2 C 



