106 F. M. GUMMOW. 



stresses in the body under great variations of temperature, 

 so that it becomes necessary to ascertain their extent in 

 order to judge if they are of sufficient importance to need 

 further consideration. 



The greatest variations of temperature to which under 

 ordinary circumstances an engineering construction would 

 be subject to, do not exceed 100° C. or 180° P. The effect 

 of that variation on a concrete-iron construction 16 inches 

 thick with two layers of round iron half an inch diameter 

 embedded 3 inches apart, would give a maximum compres- 

 sion of 14*25 lbs. per square inch on the concrete, and a 

 maximum tension of 1743 tbs. per square inch on the iron. 

 The opposite effect would be produced by an equal decrease 

 in the temperature. From this it is evident that the 

 stresses so produced are of no real consequence, and there- 

 fore unnecessary to take into calculation. 



Even under high and rapid variations of temperature no 

 disunion of the concrete and iron materially affecting its 

 carrying capacity takes place, due no doubt to the pro- 

 tection afforded the iron by the concrete. This has been 

 conclusively proved by practical tests, and by the recent 

 experiences in the great Baltimore fire, one of the largest 

 conflagrations of modern times, where the concrete-iron 

 constructions withstood the attack of a fierce fire for hours 

 without sustaining damage affecting their stability. 



2. The fact that cement is a preservative of iron against 



corrosion is so well known that it need not be further 

 dwelt upon. 



3. The adhesion of concrete to iron may be very cousider- 



able, according to the richness of the concrete mixtures, 

 and may in general be taken as equal to the shearing 

 strength of the concrete. The values ranging from 

 250 to 450 lbs. per square inch. The higher values 

 being obtained from tests with thick iron rods, and 



