March of the following year a number of preliminary experiments were
performed in order to familiarize myself with the optical arrangements.
The first experiment tried with the revolving mirror produced a deflection
considerably greater than that obtained by Foucault. Thus far the only
apparatus used was such as could be adapted from the apparatus in the
laboratory of the Naval Academy.

At the expense of $10 a revolving mirror was made, which could execute 128
turns per second. The apparatus was installed in May, 1878, at the
laboratory. The distance used was 500 feet, and the deflection was about
twenty times that obtained by Foucault.[1]

[Footnote 1: See Proc. Am. Assoc. Adv. Science, Saint Louis meeting.]

These experiments, made with very crude apparatus and under great
difficulties, gave the following table of results for the velocity of
light in miles per second:

186730
188820
186330
185330
187900
184500
186770
185000
185800
187940
------
Mean 186500 ± 300 miles per second,