Curiosities of Science. 15 



like manner, when a train previously at rest is suddenly set in 

 motion, the tendency of the passengers to remain at rest evinces 

 itself by their falling in a direction opposed to that in which 

 the train moves. 



THE LEANING TOWER OF PISA.* 



Sir John Leslie used to attribute the stability of this tower 

 to the cohesion of the mortar it is built with being sufficient to 

 maintain it erect, in spite of its being out of the condition re- 

 quired by physics to wit, that " in order that a column shall 

 stand, a perpendicular let fall from the centre of gravity must 

 fall within the base." Sir John describes the Tower of Pisa to 

 be in violation of this principle ; but, according to later au- 

 thorities, the perpendicular falls within the base. 



EARLY PRESENTIMENTS OF CENTRIFUGAL FORCES. 



Jacobi, in his researches on the mathematical knowledge of 

 the Greeks, comments on " the profound consideration of nature 

 evinced by Anaxagoras, in whom we read with astonishment a 

 passage asserting that the moon, if the centrifugal force were 

 intermitted, would fall to the earth like a stone from a sling." 

 Anaxagoras likewise applied the same theory of " falling where 

 the force of rotation had been intermitted" to all the mate- 

 rial celestial bodies. In Aristotle and Simplicius may also be 

 traced the idea of " the non-falling of heavenly bodies when 

 the rotatory force predominates over the actual falling force, 

 or downward attraction ;" and Simplicius mentions that " wa- 

 ter in a phial is not spilt when the movement of rotation is 

 more rapid than the downward movement of the water." This 

 is illustrated at the present day by rapidly whirling a pail half- 

 filled with water without spilling a drop. 



Plato had a clearer idea than Aristotle of the attractive force 

 exercised by the earth's centre on all heavy bodies removed 

 from it ; for he was acquainted with the acceleration of falling 



* When at Pisa, many years since, Captain Basil Hall investigated the 

 origin and divergence of the tower from the perpendicular, and established 

 completely to his own satisfaction that it had been built from top to bottom 

 originally just as it now stands. His reasons for thinking so were, that the line 

 of the tower, on that side towards which it leans, has not the same curvature as 

 the line on the opposite, or what may be called the upper side. If the tower 

 had been built upright, and then been made to incline over, the line of the wall 

 on that side towards which the inclination was given would be more or less 

 concave in that direction, owing to the nodding or "swagging over" of the top, 

 by the simple action of gravity acting on a very tall mass of masonry, which is 

 more or less elastic when placed in a sloping position. But the contrary is the 

 fact; for the line of wall on the side towards which the tower leans is decidedly 

 more convex than the opposite side. Captain Hall had therefore no doubt 

 whatever that the architect, in rearing his successive courses of stones, gained 

 or stole a little at each layer, so as to render his work less and less overhanging 

 as he went up; and thus, without betraying what he was about, really gained 

 stability. See Patchwork, 



