SCIENTIFIC SUMMAEY. 
253 
to its immersion in water charged ■with salts of copper and iron. From their 
position and construction, these wheels are presumed to have been worked 
as treadmills, by men standing with naked feet upon one side. — Vide The 
Artizan, November. 
MECHANICAL SCIENCE. 
Steel Boilers . — In 1859, Herr Kohn, a German engineer, jdaced in a boiler 
40 feet long, and constructed generally of iron plates, llm.m. (0’429in.) 
thick, a plate of steel only 5-|m.m. (0‘215in.) thick. After 2|- years of ser- 
vice, this plate, placed near the furnace, was found to be uninjured, whilst 
the neighbouring iron plates were much corroded. More important still, the 
quicker evaporation from the thin steel plate appears to have prevented 
incrustation. 
Cast Iron Boilers. — Mr. Zerah Colburn, C.E., read a paper before the 
British Association, chiefly to show the superiority of the Harrison boiler"^ 
over the wrought-iron boilers now generally in use. ' He points out that 
taking the tensile strength of wrought-iron plates at its minimum of 16 to 18 
tons per square inch, and that of cast-iron at 5 tons, the cast-iron spheres 
are practically as strong as wrought-iron cylinders of the same thick- 
ness. For the strength of the wrought-iron is reduced from 18 tons to 10 
tons in consequence of the riveted joints ; and, as the hollow sphere is twice 
stronger than the hollow cylinder of the same diameter and thickness, the 
cast-iron, having no joints, becomes equal in this comparison to the wrought- 
iron plate. Corrosion is the great malady of modern boilers. In the ex- 
perience of the Manchester Boiler Association with from 1,300 to 1,600 
boilers always under their care, one boiler out of every seven, and in some 
years nearly one out of every four, becomes defective by corrosion alone. On 
the other hand, the presence of a little carbon, as in cast-iron, renders it 
nearly indestructible. Hence Mr. Colburn concludes that abundant 
experience shows that cast-iron may be employed in boilers without the 
possibility of corrosion. Lastly, Mr. Colburn notes the fact observed in the 
use of cast-iron spherical boilers, that the scale is spontaneously thrown off as 
soon as formed, and blown out with the water at the end of the week. N one- 
of the iron is removed with the scale, the spheres weighing the same after 
three years’ service as when new. 
Strains of Ships . — Professor Macquorn Kankine has communicated to the 
British Association some calculations on the strains of ships afloat. He sup- 
poses the ship supported amidships on a wave crest, and dry at the ends, and 
finds that the bending action cannot exceed that due to the weight of the ship 
with a leverage of one-twentieth of her leno-th, and that the racking action 
cannot exceed about sixteen one-hundredths of her weight. Applying his 
formulae to two remarkably good examples, he gets the following results : — 
Iron Ships. 
■Wooden Ships. 
Maximum tension 
tons per square inch. 
0‘.1/5 tons per square inch. 
,, tlU'USt 
0'2y3 jy 
,, racking stress 
0'9/5 ,, ,, 
Vol. IV., page 536. 
