In the time of Isaac Newton, belief in an æther or ether which permeates all space was widespread. Rather than just dismiss the idea, Newton devised an experiment to test it,
"…since some people are of the opinion that there exists a certain aethereal medium, buy far the subtlest of all, which quite freely permeates all the pores and passages of all bodies, and that a resistance ought to arise from such a medium flowing through the pores of bodies, I devised the following experiment so that I might test whether the resistance that we experience in moving bodies is wholly on their external surface or whether the internal parts also encounter a perceptible resistance on their own surfaces."
Isaac Newton (1999) Mathematical Principles of Natural Philosophy (3rd edition, 1726): The authoritative translation (I. Bernard Cohen and Anne Whitman), University of California Press.
This was one and a half centuries before the famous Michelson-Morely experiment to measure the speed at which the earth might be moving through such an æther, and Newton's technique was comparatively crude (swinging a box with and without a load as a pendulum) – but reflected the technology available at the time.
