The letters were then transferred to the zinc by
pressure, so as to be printed from. The process, though ingenious and
of much interest at the time, has long ago been superseded by
photographic methods.
Even at this time Siemens had several irons in the fire. Besides the
printing process and the chronometric governor, which operated by the
differential movement between the engine and a chronometer, he was
occupied with some minor improvements at Hoyle's Calico Printing Works.
He also engaged in railway works from time to time; and in 1846 he
brought out a double cylinder air-pump, in which the two cylinders are
so combined, that the compressing side of the first and larger cylinder
communicated with the suction side of the second and smaller cylinder,
and the limit of exhaustion was thereby much extended. The invention was
well received at the time, but is now almost forgotten.
Siemens had been trained as a mechanical engineer, and, although he
became an eminent electrician in later life, his most important work at
this early stage was non-electrical; indeed, the greatest achievement of
his life was non-electrical, for we must regard the regenerative furnace
as his MAGNUM OPUS. Though in 1847 he published a paper in Liebig's
ANNALEN DER CHEMIE on the 'Mercaptan of Selenium,' his mind was busy
with the new ideas upon the nature of heat which were promulgated by
Carnot, Clayperon, Joule, Clausius, Mayer, Thomson, and Rankine.
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