82 - ELEMENTS OF SCIENCE 



attention, first, of the more illustrious Italian just named, 

 and afterwards of his disciple Toricelli, led to the discovery 

 that the pressure of the atmosphere will not counter- 

 balance the weight of a column of water more than 

 between 33 and 34 feet high. Experimenting with 

 mercury, the last-named observer found that after 

 filling a tube, three feet long and closed at one end, with 

 mercury, and immersing its open end in a vessel containing 

 that fluid, the mercury in the tube sank till it stood about 

 30 inches higher than the surface of the mercury in the 

 vessel. The height of the mercury sustained by atmo- 

 spheric pressure was found to be so much less than that 

 of water, on account of the much greater density of the 

 mercury. The space left above the mercury which had so 

 descended was supposed to be absolutely empty, and is 

 known (on account of the name of its observer) as the 

 " Toricellian vacuum." This space is, however, really 

 filled with the vapour of the liquid. Barometers are 

 tubular instruments which measure the weight of the 

 atmosphere by showing the height to which the pres- 

 sure of the air will raise a column of mercury (or other 

 fluid) contained within them. As it is evident that 

 the higher we ascend above the earth's surface the 

 less the weight of the atmosphere will be, it is no 

 wonder that barometers serve to indicate height, when 

 once their condition at the sea-level has been accurately 

 ascertained. Barometers only serve to indicate approach- 

 ing changes of weather in so far as such changes are 

 connected with a denser or lighter atmosphere. That 

 aeriform bodies may attain a great momentum and exer- 

 cise a vast amount of pressure is shown by the effects of 

 cyclones and hurricanes. Cyclones are rotatory move- 

 ments of air, which may be readily occasioned under 

 certain atmospheric conditions, on principles similar to 



