Proceedings of the Polytechnic Association. 811 



ferred directly through a tension-rod to the nearest end of the top 

 chord, and thus upon the pier. The other half is carried to the top 

 of the second strut from the pierj and is received at the bottom of that 

 strut by a pair of tension-rods, which subdivide this half between them ; 

 one-fourth being taken directly to the nearest pier, while the other 

 fourth is transferred to the top of the strut at the quarter span. This 

 fourth is again subdivided at the foot of this strut ; one-eighth being 

 transmitted through a tension-rod to the nearest pier, while the other 

 eighth passes to the top of the middle strut of the whole span, and is 

 received at the foot of this strut by the main tension-rods, which thus 

 transmit one-sixteenth of the original load to each pier. Thus the 

 weight at the foot of the first strut from the end of the truss, is distri- 

 buted as follows : One-half, one-fourth, one-eighth, one-sixteenth, or, 

 in all, fifteen-sixteenths of that weight reach the top of the nearest 

 pier through four converging sets of tension-rods, while the remain- 

 ing sixteenth reaches the opposite pier, after having been first brought 

 to the foot of the center strut, through the intervention of three 

 separate systems of tension-bars. With the exception of the load at 

 the foot of the center strut, which load is transmitted directly to the 

 piers, the loads at the bottom of the vertical struts are more or less 

 subdivided in the manner just described, before reaching the ends of 

 the truss." The estimated cost of this bridge, which is to be com- 

 pleted by September 1st, 1869, is $1,600,000. The chief engineer of 

 the work is Mr. Alfred Fink, and the assistant engineer, Mr. F. ' "VY. 

 Yaughan. 



REFRACTIOif OF LiGHT. 



Dr. Gladstone recently presented at the Royal Institution of London 

 the results of some new researches relating to the refractive indices of 

 different substances, showing how the index varies with temperature. 

 By means of an electric lamp he threw the yellow line in the spectrum 

 of the metal sodium on a screen, and used a hollow j)rism filled with 

 the oil of nutmeg as the refracting medium. This oil was selected 

 because its temperature is very readily changed, but any other oil will 

 answer the purpose. The prism was fixed at its angle of minimum 

 deviation, and the position of the yellow band of the spectrum was 

 marked by a strip of black paper. On stirring the oil with heated 

 metallic rods its temperature was increased, and the yellow sodium 

 line was shown to be less refracted than before. He had tried a vast 

 number of liquids, and hence concludes that all liquids have their 

 index of refraction diminished by heat. 



