﻿and Fluctuations, 



433 



Of these, Lady pit shows a difference of *025 ; when the Foul- 

 ford observations are reduced by the correction for 9 feet, they 

 show a difference of '026 ; and at pit No. 21, where the top of 

 the upcast shaft is 36 feet higher than that of the downcast, so 

 that a correction for 42 feet has to be applied, there is a differ- 

 ence of -014. 



These observations are quite in accord with those of Captain 

 Toynbee, and show beyond question a certain small relative de- 

 pression in the upcast shaft which might at first sight be attri- 

 buted to the upward motion, as opposed to the downward. But 

 we must not forget that between the columns of air in the two 

 shafts there is a very great difference of temperature and of 

 weight ; the elastic force derived from the pressure of this weight 

 is therefore less in the upcast than in the downcast ; and before 

 we can properly assign the observed barometric difference to the 

 difference of motion, we must show that it is independent of the 

 difference of weight. The following Table, which I collect from 

 the paper by Mr. Wood already referred to, seems sufficient to 

 set this point at rest *. 



Observations at Killingworth, the mean temperature of the 

 downcast being 49°, and of the upcast 147°, the sum of the 

 velocities 45*8 feet per second. 



Depth, in 

 fathoms. 



Water-gauge. 



Difference of 



weight of 



column, 



pounds per 



square foot. 



Inch. 



Pounds per 

 square foot. 



10 

 20 

 30 

 40 

 50 

 60 

 70 

 80 

 90 

 100 

 110 



•10 

 •115 

 13 

 •20 

 •25 

 •35 

 •45 

 •57 

 •70 

 •78 

 •74 



•521 



•598 

 •677 

 1041 

 1-302 

 1-822 

 2-344 

 2-968 

 3645 

 4062 

 3-854 



•769 

 1-538 

 2-308 

 3076 

 3845 

 4-614 

 5-383 

 6152 

 6-922 

 7-690 

 8-460 



Killingworth pit, in which these observations were made, had 

 the upcast and downcast in the same shaft, separated from each 

 other by an air-tight wooden partition or brattice; in this, at 

 the several heights, holes were bored, through which one leg of 

 the water-gauge (a U-shaped glass tube) could be passed. So 

 taken, and with an instrument so delicate, they show very clearly 

 that the force of the currents has very little, if any thing, to do 



* Experiments &c. pp. 20, 81, and plate 8. 



