The ability to pump water upward was attributed to the proposed fact that "Nature abhors a vacuum."  When a piston was raised, a vacuum would be produced unless the water within the cylinder rose with the piston.  Since a vacuum could not occur in nature, it was thought, the water had to rise.  In that case, it ought to have risen upward indefinitely as long as the Piston rose.  Water, however, could only be raised about thirty-three feet above its natural level.

Galileo, who accepted the vacuum-abhorrence of nature (despite his many revolutionary deeds he was surprisingly conservative in many ways), could only suppose that this "abhorrence" was limited and not absolute.  He suggested that Torricelli look into the matter.

It occurred to Torricelli that this was no matter of vacuum-abhorrence, but a simple mechanical effect.  If  the air had weight (according to Aristotle it didn't but tended rather to have "levity" and to rise) but Galileo had shown that a full balloon weight would push against the water outside the pump.  When the piston was raised, that push would force the water up with the piston.  However, suppose that the total weight for the air would only balance thirty-three feet of water.  In that case, further pumping would have no effect.  The weight of the air would push water no higher.

In 1643, to check this theory, Torricelli made use of mercury, whose density is nearly thirteen-and-a-half-times that of water.  He filled a four-foot length of glass tubing, closed at one end, put his thumb over the opening and upended it (open and down) into a large dish of mercury.  The mercury began to empty out of the tube as one might expect, but it did not do so altogether.  Thirty inches of mercury remained in the tube, supported by the weight of the air pressing down on the mercury in the dish.  The weight of the air could easily be used to account for the mercury column's remaining in place in defiance of gravity.

Above the mercury in the upended tube was a vacuum (except for a small quantity of mercury vapor).  It was the first man-made vacuum, and, thanks to the publicity given the experiment by Meraenne, is called a Torricellian vacuum to this day (Seven years later Guericke produced a vacuum on a far larger scale by pumping and did dramatic things as a result).

Torricelli noticed that the height of the mercury in the tube varied slightly from day to day and this he correctly attributed to the fact that the atmosphere positioned a slightly different weight at different times.  He had invented the first barometer.

(The weight of the stratosphere is equivalent to that of a column of mercury 760 millimeters high.  The pressure exerted by one millimeter of mercury is sometimes defined as one Torricelli, in honor of the physicist.)

The fact that air had a finite weight meant it could only have a finite height, a view confirmed by Pascal a few years later.  This was the first definite indication (aside from philosophical speculation) that the atmosphere does not extend indefinitely outward and that the depths of space must be a vacuum.   Thus, far from a vacuum being an impossibility, it is undoubtedly the natural state of most of the universe.

Questions concerning the existence of a vacuum may have seemed mystical and philosophical, but the proof of its existence led by a chain of event and reassuring to the development of the steam engine, the advent of the Industrial Revolution, and the making of our own technological society.  All resulted from the upending of a tube of mercury. Similarly, the proof for massive space-time will create a new paradigm of thinking and technology that will start a space-industry revolution.