140 SCIENCE J.Y SHORT CHAPTERS. 



THE BAROMETER AND THE WEATHER. 



THE barometer was invented by Torricelli, an Italian 

 philosopher of the seventeenth century. It consists essen- 

 tially of a long tube open atone end and closed' at the 

 other, and partly filled with mercury; but instead of being 

 filled like ordinary vessels, with the open end or mouth up- 

 wards and the closed end or bottom downwards, the bar- 

 ometer-tube is inverted, and has its open mouth down- 

 wards. This open mouth is either dipped into a little cup 

 of mercury or bent a little upwards. 



Why does not the mercury run out of this lower open end 

 and overflow the little cup when it is inverted after being 

 filled? 



The answer to this question includes the whole mystery 

 and principle of the barometer. The mercury does not fall 

 down because something pushes it up and supports it with 

 a certain degree of pressure, and that something is the at- 

 mosphere which extends all round the world, and presses 

 downwards and sideways and upwards in every direction, 

 in fact with a force' equal to its weight, i. e . , witli a pres- 

 sure equal to about 15 Ibs. on every square inch. A column 

 or perpendicular square stick of air one inch thick each 

 way,- and extending from the surface of the sea up to the 

 top of the atmosphere, weighs about 15 Ibs.; other columns 

 or sticks next to it on all sides weigh the same, and so on 

 with every portion; and all these are for ever squeezing 

 down and against each other, and, being fluid, transmit 

 their pressure in every direction, and against the earth and 

 everything upon it, and therefore upon the mercury of the 

 barometer-tube. 



We have supposed the air to be made up of columns or 

 sticks of air one inch each way, but might have taken any 

 other size, and the weight and pressure would be propor- 

 tionate. Now mercury, bulk for bulk, is so much heavier 

 than air, that a stick or column of this liquid metal about 

 30 inches high weighs as much as a stick or column of air 

 of same thickness reaching from the surface of the earth to 

 the top of the atmosphere; therefore, the 30-inch stick of 



