ERA OF COPERNICUS, TYCHO BRAKE, KEPPLER, AND GALILEO. 23 



which would, therefore, fall at that distance behind it. This argument was employed by 

 Ptolemy, who stated that, if the earth revolved with great rapidity from west to east, it 

 would leave behind it the clouds, birds flying in the air, and, generally, all objects sus- 

 pended in the atmosphere. The answer to this is, that a falling body will partake of a 

 rectilinear and circular motion ; the former tending to the centre of gravity, the latter 

 proceeding in the direction of the circumference described by the point from which it 

 falls. Gassendi tried experiments in the harbour of Marseilles, and proved, what every 

 one now knows, that a stone dropped from the mast of a vessel in full sail will partake 

 of the advance of the mast, and fall at its foot, as though the vessel were at rest. It 

 may also be proved, that a stone falling from a considerable elevation, so far from 

 being left behind during its descent, will fall in advance of the base of the perpen- 

 dicular, upon the theory of the earth's rotation. Let the circle E be the equatorial 

 circumference of the earth, the line T a tower perpendicular to c the centre, and the 

 circle M will then be the circumference described by the summit of the tower s, in the 

 course of one rotation of the earth upon its axis. If we suppose the 

 base of the tower, b, to pass to c, the summit, s, will, in the same 

 time, pass to , and, this being the larger arc, it follows that the sum- 

 mit must travel faster than the base. Assuming, then, the earth's 

 rotation eastward, a stone dropped from the summit will leave it with 

 its momentum, and will move faster eastward, through the whole of 

 its descent, than the base. The result will be, that it will deviate 

 from the plumb-line, and reach the ground a little to the east of c, 

 the foot of the perpendicular. Experiment has confirmed the accuracy of this reason- 

 ing, though obviously a very difficult matter to test, owing to the comparatively small 

 height of buildings suitable to the purpose. Newton first threw out the idea, and cal- 

 culated that a ball would deviate about half an inch from the plumb-line, to the east, from 

 the height of three hundred feet. Thirty balls descended from the height of two hundred 

 and thirty-five feet, in St. Michael's tower Hamburg, and deviated from the perpendicu- 

 lar four lines eastward, swerving also one line and a half southward, owing probably 

 to a current of air in the tower. The experiment was, therefore, repeated in a coal-pit, 

 in the county of Mark, two hundred and sixty feet deep. There the balls fell five lines 

 eastward of the perpendicular, but neither northward nor southward, so that the theory of 

 the earth's motion on its axis may now be said to have received a complete and sensible 

 confirmation. 



The reason which led Tycho to reject the doctrine of the earth's orbital motion was 

 of great force in his time. Supposing the earth to revolve round the sun, two points of 

 the orbit will be distant from each other by the whole diameter of the orbit ; yet lines 

 drawn from those points to the nearest fixed star discovered no appreciable angle, or 

 annual parallax. It is sufficient to reply now, that the distance of the stars is so great as 

 to render insensible the diameter of the earth's orbit ; but, previously to the invention of the 

 telescope, this answer could not be deemed so satisfactory. To the naked eye the stars of 

 the first magnitude present a diameter equal to two minutes of space. The telescope has 

 shown this to be an optical delusion, that no star has an apparent diameter of a second. 

 But Tycho, knowing that if the earth moved, its change of place from one extremity of 

 the diameter of its orbit to another produced no sensible alteration in the place of the 

 stars, so that they must be at an enormous distance, and yet seeing them present a dia- 

 meter to the eye varying from a quarter to as much as two minutes, had little option but 

 to assign a magnitude utterly inconceivable to the nearest fixed star, or reject the theory 

 of the earth's motion in space. He chose the latter alternative; and, though undoubtedly 

 it would have been a wiser course to have paused before coming to such a conclusion, 



