of the Royal Society of Edinburgh. 351 



the accumulated deposit. To prevent this process taking place 

 here, the case has been made carefully air-tight against mode- 

 rate pressure, excepting in one place, where a short tube is fixed 

 in an opening from which it projects externally about two inches. 

 On this projection a half distended air-bag is made fast *. It 

 follows from this arrangement, that when a contraction takes 

 place within the case, the pressure of the external air will com- 

 press the bag, and make a portion of its contents enter the case 

 to make up for it ; and when, on the contrary, an expansion 

 takes place, the expressed air will enter the bag and distend it ; 

 in this way, if the capacity of the bag be great enough, no fo- 

 reign air (if it may be so termed) can enter the case, but the 

 equilibrium will be kept up by a circulation of the same air be- 

 tween the bag and the case, like the mercury in the basin and 

 tube of a barometer, and no dust can ever be added to the quan- 

 tity originally shut up with the clock. It is evident that this 

 must conduce essentially both to regular performance and to the 

 durability of the clock, and as the application of the contrivance 

 occasions little expense or inconvenience, there is no reason why 

 it should be omitted in any observatory clock-case. 



* The air-bag is concealed within the pediment of the top of the clock-case. 



