BREWSTER, DAYID. 



BRIDGES. 



79 



Society of Edinburgh, of which he was for 

 many years Vice-President. He had already 

 (in 1816) received half the French Institute's 

 physical prize bestowed for two of the most 

 important discoveries made in Europe during 

 the two preceding years. In 1819, in con- 

 junction with Professor Jamieson, he under- 

 took the editing and publication of the Edin- 

 burgh Philosophical Journal, which he subse- 

 quently continued alone, through sixteen vol- 

 umes, under the title of the Edinburgh Journal 

 of Science. He was also, for many years and 

 until his death, one of the editors of the Lon- 

 don, Edinburgh, and Dublin Philosophical 

 Magazine. In 1825 he was elected a corre- 

 sponding member of the Institute of France ; 

 in 1830 he received the Royal Medal from the 

 Royal Society of London, for his further dis- 

 coveries in the polarization of light, and the 

 same year received from William ' IV. the 

 Guelphic Order, and the following year was 

 knighted. From 1827 to 1833 he was engaged 

 in investigations on the best methods of illu- 

 mination for light-houses, and published a 

 treatise on the subject, which led to the great 

 improvements that have been made in light- 

 houses since that time. In 1833 he was chosen 

 principal of the United Colleges of St. Leon- 

 ard's and St. Salvator at St. Andrew's, which 

 position he continued to hold till his death. 

 In 1859 he was chosen as one of the eight 

 foreign associates of the Institute of France, 

 and, the same year, Principal and Vice-Chan- 

 cellor of Edinburgh University. He was one 

 of the prime movers in the organization of the 

 " British Association for the Advancement of 

 Science," and attended its annual sessions very 

 regularly till within the last two years of his 

 life. In 1857 he presided over its session at 

 Edinburgh. Though not the discoverer of the 

 principle of the stereoscope, he was the first 

 to reduce it to practical uses. His improve- 

 ments upon the construction of microscopes 

 and telescopes were numerous and of great 

 value to science. Among his other contribu- 

 tions to practical science were the initiation of 

 the Bude Light, and his demonstration of the 

 utility of dioptric lenses and of zones in light- 

 houses. Few men were as familiar as he with 

 the profoundest secrets of natural science, yet 

 no man of his time did as much to popularize 

 it, and excite a thirst for scientific knowledge 

 in the hearts of even the illiterate. His works 

 on the "Kaleidoscope," "The Stereoscope," 

 "Natural Magic," "Martyrs of Science," 

 "More Worlds than One" (a reply to Dr. 

 Whewell's "Plurality of the Worlds"), his ad- 

 mirable " Life of Newton," and that long suc- 

 cession of brilliant and charming articles on 

 scientific subjects in the North British Re- 

 view, and Popular Science Magazine, gave 

 evidence of his disposition in this respect. 

 He retained his intellectual and much of his 

 physical vigor to the close of his life, and, 

 though in his eighty-sixth year, came forward 

 in the summer of 1867 and took an active part 



in exposing the literary forgery of the pretend- 

 ed correspondence between Sir Isaac Newton 

 and Pascal. Sir David was the last as he was 

 one of the ablest of the great physicists of the 

 last generation. 



BRIDGES. Quincy Railway Bridge. This 

 bridge, crossing the Mississippi, and connect- 

 ing with the Chicago, Burlington, Quincy, 

 and the Hannibal and St. Joseph Railways, 

 having just been completed, was opened for 

 general traffic on the 7th of November. The 

 design and execution were intrusted to Mr. T. 

 0. Clarke, of Chicago. The part of the bridge 

 crossing the main branch of the river, consists 

 of two draw spans of 180 feet each. 11 spans of 

 157 feet each, 2 spans of 250 feet, 3 of 200 feet, 

 1,400 feet of embankment and trestle-work, and 

 a smaller bridge 613 feet long, divided into one 

 draw span 160 feet long, and four openings of 

 85 feet each, making the total length of the 

 bridge and embankment from one railway to 

 the other nearly two miles. The bridge is ele- 

 vated 10 feet above high-water, and 20 feet 

 above low-water mark, and the iron superstruc- 

 ture, on the Pratt-truss system, is supported 

 on stone piers and abutments. The work was 

 completed with unusual expedition, the first 

 stone having been laid on September 25, 1867, 

 and the bridge finished on the 5th of August, 

 1868. The total cost was, $1,500,000. 



The bridge is so proportioned that, under the 

 greatest loads, the utmost strain that can be 

 brought upon it is 7.500 tons to the inch, the 

 ultimate strength being 60,000 Ibs. to the inch. 

 In selecting the iron employed, all bars were 

 tested up to a strain of 23,600 Ibs. to the inch. 

 The average quality of the material resisted a 

 tension of 28,000 Ibs. to the inch before a per- 

 manent set took place; some of the specimens 

 broke under a strain of 80,000 Ibs. The bridge 

 before it was opened was subject to the fol- 

 lowing tests : Three of the heaviest locomo- 

 tives upon the Chicago, Burlington, and Quincy 

 Railroad, weighing together 300,000 Ibs., were 

 coupled together, and placed upon each of the 

 250-feet spans, in succession, and the deflection 

 was accurately observed. The maximum de- 

 flection under this load was 2--/fo inches, 

 being less than one-sixteenth of the previous 

 calculation. The same load was then placed 

 upon a span 157 feet; the deflection in this 

 case was 1^^ inches. The three locomotives, 

 still coupled together, were then run backward 

 and forward over the 157-feet span, at rates 

 varying from 10 to 16 miles an hour. The de- 

 flection produced was'l T Vcnr inches, or T fg- of 

 an inch more than was produced by the load 

 when at rest. In every case the structure re- 

 sumed its form when the engines were with- 

 drawn, no permanent set being visible. On 

 the 157-feet span the load applied was equiva- 

 lent to a stress of 9,000 Ibs. per square inch on 

 the wrought iron, and 10,200 Ibs.on the cast iron, 

 or about one quarter more than the passage of 

 the heaviest freight trains over the bridge can 

 produce. 



