Browning's New Aneroid Barometer. 39 



fluid in the open cistern will have to bear more weight and a 

 portion of it will be shoved up into the vacuum chamber, 

 causing the column of water , or mercury, as the case may be, 

 to rise in the tube. If the air grows lighter, the pressure on 

 the cistern will be less, and a corresponding portion of the 

 fluid will fall out of the tube into the cistern until a balance 

 is obtained. 



That which is necessary to this kind of barometer is a 

 vacuum chamber in which a fluid can rise or fall. The vacuum 

 chamber is formed of glass on all sides but one, and that side 

 is formed by the fluid, which contracts the chamber by rising, 

 and enlarges it by falling. If, therefore, we have a vacuum 

 chamber that permits one or more of its sides to rise and fall 

 as the pressure upon it changes, we may employ it as a baro- 

 meter, whatever may be its form or construction ; but some 

 forms and some constructions will be much more sensitive than 

 others. Suppose we tied a bladder over the mouth of a tum- 

 bler and then exhausted part of the air, we should find the 

 bladder pressed down in the middle. If we put the tumbler 

 so arranged under an air-pump and began to exhaust the air, 

 the bladder would begin to rise. The air inside the tumbler 

 being elastic, would occupy more and more space as the 

 pressure upon it was reduced. Thus, in a rough way, and only- 

 capable of showing considerable changes, we should have 

 made a barometer out of a tumbler and a bladder. 



Suppose that, desiring more accuracy, we constructed a 

 flat cylindrical chamber of thin elastic metal, and then ex- 

 hausted nearly all the air. Here we should have a more sen- 

 sitive instrument ; but it would vary so little in its dimensions 

 under such changes of pressure as take place at the earth's 

 surface, that we should not be able to see that any variation 

 occurred. We, therefore, must go a step further. In the first 

 place, we may fix our little vacuum chamber by the centre of 

 its lower surface to a firm stand, then we may attach a spring 

 to the upper surface, which counteracts a certain portion of 

 the atmospheric pressure. The air with all its weight presses 

 the chamber so as to make it collapse. The small quantity 

 of air left in the chamber, by its elasticity does a little to 

 enable the walls of the chamber to resist the air's force, and 

 the spring exercises a stronger action of the same kind. The 

 result is, that when the air has produced a certain amount 

 of collapse, a balance is obtained. That is to say, when the 

 spring has been stretched to a certain point, and the minute 

 quantity of air inside the chamber has been compressed to a 

 certain extent, the air pressure has done its work. Under these 

 circumstances, a little more air pressure produces a little more 

 collapse, and a little less air pressure allows the spring and 



