NATURAL PHILOSOPHY. 147 



a wide, deep pit with a plummet suspended from its mouth at the earth's 

 surface. The plumb-line will be vertical only when in the center of the pit 

 (or shaft, it is called in connection with mines), because it will there be 

 equally attracted in every direction. If carried round the side of the pit, 

 the line will be so deflected from the vertical as to cause all the lines, if 

 produced upwards, to meet in a point above the earth's surface; and such 

 are the phenomena discovered by the geodetical engineers of Russia. The 

 pit, it is true, is closed at its mouth, and no plumb-line can be let down 

 into it, but the spirit-level, being always at right angles to the plumb-line, 

 discloses the fact as clearly to the mind as the open pit would to the eye. 

 Now, whether the Cowhythe deflection is to be accounted for by a compar- 

 ative vacuum on the north under the Moray Firth, or by some unknown 

 mass oi' extraordinary density on the south, or partly by both, is the prob- 

 lem to be solved, and, doubtless, it will ultimately be solved by the staff of 

 astronomical obse- vers and computers under !Sir Henry James. The gen- 

 eral result of the investigations thus far, may be briefly stated to be a. 

 diminution of the deflection as the observers proceed southwards, but 

 how far it may extend is of course at present unknown. 



THE BAROMETER AS AN INDICATOR OF THE EARTH'S ROTATION. 



Mr. Pliny Earle Chase, in a paper recently read before the Ameri- 

 can Philosophical Society On the Barometer as an Indicator of the 

 Earths Iiotation and the Sun's Distance; sets forth the following 

 views : The existence of daily barometric tides has been known for 

 more than 150 years, but their cause is still a matter of dispute. It 

 is evident that they cannot be accounted for by variations of tempera- 

 ture, for 1st, their regularity is not perceived until all the known 

 effects of temperature have been eliminated ; 2d, they occur in all 

 climates, and at ail seasons ; 3d, opposite effects are produced at dif- 

 ferent times, under the same average temperature. Thus at .St. Hel- 

 ena, the mean of three years 1 hourly observation gives the following 

 average barometric heights : 



From O h to 12 h 28'2801 in. From 18 h to 6 h 28'2838 in. 



" 12 !l to 0" 28-2761 " " G h to 18 h 28-278-1 " 



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 ; 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 revo- 

 lution, each particle of air has a velocity in the direction of its orbit, 

 varying at the equator from about 05,000 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 pro- 

 duced by each in 24 hours, the interval that elapses between two suc- 

 cessive returns of any point to the same relative position with the sun. 

 The force of rotation 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 ^VsWVo' 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. 



