Another excellent micrometer, originally based on a model by Clark of Cambridge, Massachusetts, has been largely used by Burnham and others in America.
Instruments have been invented by Alvan Clark and Sir Howard Grubb for measuring with the spider-line micrometer angles which are larger than the field of view of the eyepiece.
Clark in Dict.
In 1967, Colin Clark estimated that the minimum space necessary to feed a person was 27 square kilometers, a then-optimistic figure that had not been proven in commercial practice or even large-scale experiment.
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