CH. XV. 



THE THERMOMETER. 



117 



mark off inches and parts of inches on the side of the tube, so 

 as to reckon how much the mercury rose and fell each day. 



This was the beginning of the barometer, by ($> 

 which we measure the weight of the atmosphere. 

 It was a long time before people would believe 

 that anything so invisible as air could affect the 

 mercury, but this was at last clearly proved by 

 a man named Perrier, who carried a baro- 

 meter to the top of a mountain called the Puy 

 de Dome, in Auvergne. As the summit of a 

 mountain reaches to a great height in the atmo- 

 sphere, it has, of course, less air resting upon 

 it than the valley below has, and so the mer- 

 cury when carried to this height not being 

 pressed so much up the tube, fell nearly 3 

 inches, and then rose again gradually as M. 

 Perrier came down into the valley below, where 

 there was a greater weight of air. This ex- 

 periment, which was suggested by the famous 

 French writer Pascal, con firm ed Torric elli's theory , 

 and proved beyond doubt that it was the weight 

 of the air which caused the mercury to rise. 



If now, after reading this account, you go 

 and look at an ordinary upright barometer (Fig. 

 1 6), you will perhaps be puzzled by finding it 

 all enclosed in wood, and you will ask how the 

 air can get to the mercury to press it down ; F:G , 6 

 but if you look carefully at the wooden box at Ordinary upright 



... , Barometer. 



the bottom, you will find a small hole .in the A> Wood covering 

 wood, often having a small plug of paper in it cup of mercury. 



11 i ,1 i i B, Hole through 



to keep out the dust, and through tins hole, which air acts . 



even stuffed up as it is, the pressure of the air 



can act. The space between the top of the column of 



