ASTRONOMY AND IJAIfO.M KTKMC HYPSOMETRY. 555 



ing and descending high mountains. P^very large change in the height of 

 the mercury will have a corresponding effect on the rate of the chronometer. 



From some experiments made by Prof. W. C. Bond at the Cambridge 

 Observatory in 1856, it was found that when at a mean pressure of 29'. 90 

 a chronometer which had a daily gaining rate of one second, when put 

 under the glass receiver of an air-pump and the pressure brought down to 

 20'. 00, the daily rate increased to nine seconds. 



It is yet to be determined whether or not the decrease in the force of 

 gravity from the sea level to the very high altitudes to which chronometers 

 have been carried may not have some sensible effect on the delicate moving 

 portions of a chronometer. 



