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the advantage of employing stars in the zenith
for determining the fundamental constants
of astronomy. He devised a reflex zenith
tube , in which the zenith point was deter-
mined by reflection from a surface of mer-
cury. The design was so simple, and seemed
so perfect, that great expectations were en-
tertained. But unaccountable variations com-
parable with those of the transit circle ap-
376