Raul Rojas' 2014 paper provides the first comprehensive description of the Z1, the mechanical
computer built by the German inventor Konrad Zuse in Berlin from 1936 to 1938. The arxiv
paper describes the main structural elements of the machine, the high-level
architecture, and the dataflow between components. pdf 
A view of the reconstructed Z1 in Berlin. This from the 360° view available at the Konrad Zuse Internet Archive. pano
The information for this article was extracted from careful study of the blueprints drawn
by Zuse for the reconstruction of the Z1 for the German Technology Museum in Berlin,
from some letters, and from sketches in notebooks. site
The Z1 was a mechanical but also a surprisingly modern computing machine. It was based on binary numbers, used a floating-point representation and could perform the four basic arithmetic operations. The program sequence was read from a punched tape and the results stored to or read from a 16 words memory. The machine cycle was 4 Hz.
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A Zuse exhibit inspired CS professor Harry Porter to build a working computer out of relays using Zuse's design for the adder. See Harry Porter's Relay Computer