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THE POPULAR SCIENCE MONTHLY. 



case that the study of carbon compounds 

 is being so vigorously prosecuted. ... As 

 to the value of the work, I believe that ev- 

 ery fact honestly recorded is of value." No 

 unprejudiced reader can but be struck with 

 the improvement in quality which is mani- 

 fest in the majority of the investigations 

 now published. The great outcome of the 

 labors of carbon chemists has been the es- 

 tablishment of the doctrine of the structure. 

 That doctrine has received the most power- 

 ful support from the investigation of physi- 

 cal properties, and it may almost, without 

 exaggeration, be said to have been rendered 

 visible in Abney and Festing's infra-red 

 spectrum photographs. 



Limits of Stress on Iron Bridges.— Ad- 

 dressing the Mechanical Science Section of 

 the British Association, Mr. B. Baker spoke 

 of the want of understanding among engi- 

 neers regarding the admissible intensity of 

 stress on iron and steel bridges, concerning 

 which " at the present time absolute chaos 

 prevails. The variance in the strength of 

 existing bridges is such as to be apparent to 

 the educated eye without any calculation. 

 ... It is an open secret that nearly all the 

 large railway companies arc strengthening 

 their bridges, and necessarily so, for I could 

 cite eases where the working stress on the 

 iron has exceeded by two hundred and fifty 

 per cent that considered admissible by lead- 

 \n" American and German builders in simi- 

 lar cases. ... In the present day engineers 

 of all countries are in accord as to the prin- 

 ciples of estimating the magnitude of the 

 stresses on the different members of a. 

 structure, but not so in proportioning the 

 members to resist those stresses. The prac- 

 tical result is, that a bridge which would be 

 passed by the English Board of Trade would 

 require to be strengthened five per cent in 

 some parts and fifty per cent in others be- 

 fore it would be accepted by the German 

 (iovernmcnt, or by any of the leading rail- 

 way companies in America." This unde- 

 sirable state of affairs arises from the fact 

 that " many engineers still persistently ig- 

 nore the fact that a bar of iron may be 

 broken in two ways — namely, by the single 

 application of a heavy stress, or by the re- 

 peated application of a comparatively light 

 Btress. An athlete's muscles have often 



been likened to a bar of iron, but, if ' fa- 

 tigue' be in question, the simile is very 

 wide of the truth. Intermittent action — the 

 alternative pull and thrust of the rower, or 

 of the laborer turning a winch — is what the 

 muscle likes and the bar of iron abhors. 

 From tests made several years ago by royal 

 commissioners, the deduction was made that 

 " iron bars scarcely bear the reiterated ap- 

 plication of one third the breaking weight 

 without injury, hence the prudence of al- 

 ways making beams capable of bearing six 

 times the greatest weight that could be laid 

 upon them." Hundreds of existing railway- 

 bridges which carry twenty trains a day with 

 perfect safety would break down quickly 

 under twenty trains an hour. Although 

 many more experiments are required before 

 universally acceptable rules can be laid 

 down, " I have thoroughly convinced myself 

 that, when stresses of varying intensity oc- 

 cur, tension and compression members should 

 be treated on an entirely different basis." 



Some Aspeets of Heredity. — Mr. Francis 

 Galton spoke, in the Anthropological Section 

 of the British Association, from his re- 

 searches in family histories and records, on 

 types and their inheritance. He discussed 

 the conditions of the stability and insta- 

 bility of types, and urged the existence of a 

 simple and far-reaching law governing the 

 hereditary transmission. From experiments 

 he had made several years before on the 

 produce of seeds of different size but the 

 same species, it appeared that the offspring 

 did not tend to resemble their parent-seed 

 in size, but to be always more mediocre 

 than they — to be smaller than the parents 

 if the parents were large, to be larger than 

 the parents if the parents were very small. 

 The special subject of this paper was hered- 

 itary stature, where a similar law seemed to 

 prevail. His data consisted of the heights 

 of nine hundred and thirty adult children 

 and their parentages, two hundred and five 

 in number. The cliild inherits partly from 

 his parents, partly from his ancestry. Speak- 

 ing generally, the further his genealogy goes 

 back, the more numerous and varied will his 

 ancestry become, until they cease to differ 

 from any equally numerous sample taken at 

 hap-hazard from the race at large. Their 

 mean stature will then be the same as that of 



