THE BAROMETER, 



tion as the fluid is lighter, will the change in the height of the column, by a 

 given change in the pressure of the atmosphere, be greater ; but there are diffi- 

 culties of a different kind which altogether preclude the use of other fluids. 

 The lighter liquids are much more susceptible of evaporation, and the surface 

 of the liquid in the tube being relieved from the atmospheric pressure, offers no 

 resistance to the process of evaporation. The consequence is, that any liquid, 

 except mercury, would produce a vapor, which, occupying the top of the tube, 

 would press by its elastic force upon the surface, and co-operate with the 

 weight of the suspended column in balancing the atmospheric pressure. Even 

 from mercury we have reason to know that a vapor rises, which is present in 

 the upper part of the tube ; but this pressure exerts no power which can intro- 

 duce inaccuracy to any sensible extent into our conclusions. 



A form is sometimes adopted called the diagonal barometer, for the purpose 

 of increasing the range of the mercury in the tube. This is represented in 

 fig. 4, where A C B represents the barometer tube. 



C is a point at a distance above the surface of the mercury in the cylinder 

 less than the height of twenty-eight inches. The space C D includes the range 

 which the mercury would have if the tube were vertical ; but at C the tube is 

 bent obliquely in the direction C B, having a sufficient length to bring the ex- 

 tremity B to the same level as D. The mercury, which, had the tube been 

 vertical, would range between C and D., will now have its play extended through 

 the greater space C B ; consequently the magnitude of any part, however 

 small, will be increased in the proportion of the line C D to the line C B. 

 Thus, if C D be four inches, and C B twelve inches, then every change in the 

 position of the surface of the mercury produced by a change in the atmospheric 

 pressure, will be three times as great in the diagonal barometer as it would be 

 in the vertical one. 



Fig. 4. 



Fig. 5. 



32_ 





Another contrivance fcr enlarging the scale, which is more frequently used, 

 and for common domestic purposes attended with some convenience, is repre- 

 sented in fig 5. This is called the wheel barometer. The barometric tube is 

 here bent at its lower extremity B, and turned upward toward C. The atmo- ' 

 spheric pressure acts upon the surface F, and sustains a column of mercury in | 



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