Now, in order to define the motion of evey part of the fluid, an infinite number of line would have to be drawn at indefenitely small intervals ; but since the description of such a system of lines would involve continual reference to the theory of limits, it has been thought better to suppose the lines drawn at intervals depending on the assumed unit, and afterwards to assume the unit as small as we please by taking small submultiple of the stamdard unit.

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http://electrogravityphysics.com/wp-content/uploads/On-Faradays-Lines-of-Force.pdf

(5)の部分です