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VI. On the Barometer as an Indicator of the Earth* s Rotation 

 and the Sun's Distance, By Pliny Earle Chase*. 



THE existence of daily barometric tides has been known for 

 more than a hundred and fifty years, but their cause is 

 still a matter of dispute. It is evident that they cannot be 

 accounted for by variations of temperature, for (1) their regula- 

 rity is not perceived until all the known effects of temperature 

 have been eliminated ; (2) they occur in all climates, and at all 

 seasons; (3) opposite effects are produced at different times, 

 under the same average temperature. Thus at St. Helena the 

 mean of three years' hourly observation gives the following 

 average barometric heights : — 



h h in. h h in. 



From to 12 28-2801 From 18 to 6 28*2838 

 From 12 to 28*2861 From 6 to 18 28-2784 

 The upper lines evidently embrace the coolest parts of the day, 

 and the lower lines the warmest. Dividing the day in the first 

 method, the barometer is highest when the thermometer is 

 highest j but in the second division the high barometer prevails 

 during the coolest half of the day. 



On account of the combined effects of the earth's rotation and 

 revolution, each particle of air has a velocity in the direction of 

 its orbit, varying at the equator from about 65,000 miles per 

 hour at noon, to 67,000 miles per hour at midnight. The force 

 of rotation may be readily compared with that of gravity by 

 observing the effects produced by each in twenty-four hours, the 

 interval that elapses between two successive returns of any point 

 to the same relative position with the sun. The force of rota- 

 tion producing a daily motion of 24,895 miles, and the force of 

 terrestrial gravity a motion of 22,738,900 miles, the ratio of the 

 former to the latter is 2AWo 5 oo> or '00109. This ratio repre- 

 sents the proportionate elevation or depression of the barometer 

 above or below its mean height that should be caused by the 

 earth's rotation, and it corresponds very nearly with the actual 

 disturbance at stations near the equator. 



From h to 6 h the air has a forward motion greater than that 

 of the earth, so that it tends to fly away ; its pressure is there- 

 fore diminished, and the mercury falls. From 6 h to 12 h the 

 earth's motion is greatest ; it therefore presses against the lag- 

 ging air, and the barometer rises. From 12 h to 18 h the earth 

 moves away from the air, and the barometer falls ; while from 

 18 h to 24 h the increasing velocity of the air urges it against the 

 earth, and the barometer rises. 



If the force of rotation at each instant be resolved into two 

 components, one in the direction of the radius vector, and the 

 * From Silliman's American Journal for Mav 1864. 



