388 



effect of curves involving changes of direction of the road, the velocity 

 of the transit, and the distance between the rails ; but, for the reason 

 already stated, not comprising the radii of the curves. 



Although the radii of the curves do not form a constant element of 

 the estimate of the mechanical power necessary to work the road, 

 nevertheless they are of material consequence, as far as regards the 

 safety of the transit. Although a short curve with a great resistance 

 may be moved over with the same expenditure of mechanical power 

 as a long curve with a long radius, yet, owing to the intensity of the 

 pressure of the flange against the rail, the danger of the trains run- 

 ning off the road is increased : hence, although sharp curves cannot 

 be objected to on the score of loss of power, they are yet highly ob- 

 jectionable on the score of danger. 



In the present paper, the author has confined himself to the ana- 

 lytical formulae expressing various mechanical effects of the most 

 general kind ; the coefficients and constants being expressed merely 

 by algebraical symbols : but he states that he has made an extensive se- 

 ries of experiments within the last few years, and has also procured the 

 results of experiments made by others, with a view to determine the 

 mean values of the various constants in the formulae investigated in 

 this paper. He has also, with the same view, made numerous ob- 

 servations in the ordinary course of transit on railways ; and he an- 

 nounces his intention of soon laying before the Society, in another 

 paper, the details of these experiments, and the determination of the 

 mean values of these various constants, without which the present 

 investigation would be attended with little practical knowledge. 



A paper was also read, entitled " Register of the State of the 

 Barometer and Thermometer kept at Tunis, during the years 1829, 

 1830, 1831 and 1832." Presented by Sir Thomas Reade, His 

 Majesty's Agent and Consul General at Tunis. Communicated by 

 P. M. Roget, M.D., Sec. R.S. 



The observations here registered are those of the thermometer at 

 9 a.m., at noon, and at 6 p.m., and the points of the wind, and height 

 of the barometer for each day of the abovementioned years. 



May 5, 1836. 



FRANCIS BAILY, Esq., Vice-President and Treasurer, in the 



Chair. 



Edward Burton, Esq., William Sands Cox, Esq., and Captain 

 Thomas Locke Lewis, R.E., were elected Fellows of the Society. 



A paper was in part read, entitled " On the Optical Phenomena of 

 certain Crystals." By Henry Fox Talbot, Esq., F.R.S. 



May 12, 1836. 



The Rev. WILLIAM WHEWELL, M.A., Vice-President, in the 



Chair. 



The reading of a paper, entitled " On the Optical Phenomena of 



