160 ATMOSPHERIC PRESSURE. 



water bath. Tfie water in the tube came to rest at 

 a height of 34 feet. ) The water column was 13.6 times 

 as high as the mercury column, but as the specific gravity 

 of mercury is 13.6, the weights of the two columns were 

 equal. Experiments with still other liquids gave corres- 

 ponding results, all of which strengthened the theory that 

 the supporting force is due to the weight of the atmos- 

 phere, and left no doubt as to its correctness. 



277. Pressure Measured in Atmospheres. 



A gas or liquid which exerts a force of fifteen pounds upon 

 a square inch of the restraining surface is said to exert a 

 pressure of one atmosphere. A pressure of 60 pounds to 

 the square inch, or 4 Kg. to the sq. cm., would 

 be called a pressure of four atmospheres. 



278. The Accuracy of a Barom- 

 eter. The accompanying figure represents 

 the simplest form of the barometer. The in- 

 strument's accuracy depends upon the purity 

 of the mercury, the accuracy of measuring the 

 vertical distance from the level of the liquid 

 in the cistern to that in the tube, and the 

 freedom of the space at the top of the tube 

 from air and moisture. In delicate observa- 

 tions allowance must be made for differences 

 of temperature. In technical language, 

 "The barometric reading is corrected for 

 temperature." 



279. The Utility of a Barometer. 



This instrument's efficiency depends upon 



the fact that variations in atmospheric pres- FIG. 95. 



