Otago Institute. 477 



planetary character was placed beyond doubt. Subsequent observations have 

 proved that it is to this planet, since called Neptune, that the perturbations 

 already mentioned must, according to the Newtonian law of gravity, be 

 assigned. This narrative is calculated to stimulate the study of science. It 

 shows what human perseverance governed by science can effect. Not that it 

 is given to many men to discover a planet or a star ; but science has numerous 

 fields of inquiry which are open to the aspiring student, and in which every 

 one may hope to discover something new and useful to his fellow men. 



Scarcely inferior to this as a scientific discovery by the mere force of 

 reasoning, and superior in practical results, is that which is described by 

 Tyndall in one of his admirable lectures, namely, the discovery or rather 

 invention of the barometer, which was arrived at by a process of scientific 

 reasoning. It grew out of the common pump. About 1632 the Grand Duke 

 of Tuscany was desirous of improving the public gardens of J'lorence, and in 

 order to raise water to a considerable height he ordered some large pumps to 

 be made. When they were set to work it was found that the water would 

 not rise above 32 feet. What could be the cause of this 1 The hypothesis 

 then current was, that nature abhorred a vacuum. Had her supposed 

 abhorrence a limit % The problem was submitted to Galileo, but he was then 

 in an ill humour in consequence of the persecutions of the Church for his 

 heretical and unscriptural doctrine that the earth moves round the sun, and 

 he answered sulkily that he supposed that nature only abhorred a vacuum up 

 to 32 feet. The real meaning of his answer was that he was unable to solve 

 the problem. But it was taken up by his pupil, Torricelli. He assumed that 

 the water could not move up the exhausted cylinder of the pump without the 

 application of some external force, and he conjectured that that force was the 

 weight of the column of the atmosphere. Galileo had previously proved that 

 air is not destitute of weight. Torricelli then reasoned thus : If the weight of 

 the atmospheric column be the exact equivalent of the weight of 32 feet of 

 water, then, inasmuch as mercury is about thirteen times as heavy as water, 

 the column of air ought to support about 30 inches of mercury. This grand 

 scientific conception being once generated the proof was easy. Torricelli took 

 a glass tube about three feet long, closed — ^that is, hermetically sealed — at one 

 end ; into the open end he poured mercury until it was full, then closing the 

 orifice with his finger or thumb he inverted the tube and plunged his hand 

 into an open basin of mercury, upon the surface of which the external air 

 could freely act ; he then removed his hand, and you may judge of his delight 

 when he found that the mercury fell to about 30 inches and there stopped. 



This experiment was soon followed by another, which confirmed Torricelli's 

 theory (if indeed it needed confirmation). The French philosopher, Blaise 

 Pascal, reasoned thus : If Torricelli be right, if the water in the pump and 



