'The simile isn't important to the tale, nor to us. Indeed, it is possibly one of those Kipling technicalities which do not bear too close an examination. A piston-head did not have packing when the tale was written. In the very early days of steam, it is said that hemp was used to make a piston steam-tight - but that was in the 1830s and `40s. By the time of the tale, cast iron would have been used to make piston rings. Nor would the piston, being enclosed in a cylinder closed at both ends, have been visible for the Chief Engineer to determine whether or no it was 'sweating'.

However, it may be fair to suggest that in engines with very low steam pressure, at the end of the working stroke, the temperature of the cylinder walls would have been low enough to cause a very fine mist of minuscule water droplets, more a miasma, to have formed on those walls, and hence been transferred to the piston rings on the piston.

The Chief Engineer would have been aware of this phenomenon. But a monumental sweat it certainly was not. Had it been such, the resulting water would have blown off the cylinder head, since water is incompressible. The best that can be said is that Kipling's Chief Engineer knew how to spin a tale.' [A.J.W.W.]
See also the notes by Al;astair Wilson on two tales in The Day's Work; “The Ship that Found Herself” (page 84, line 11), and “The Devil and the Deep Sea" (page 152, line 17)

'And Sampson took hold of the two middle pillars upon which the house stood , and on which it was borne up … And he bowed himself with all his might; and the house fell upon the lords and upon all the people that were therein…'