350 Notice regarding a Time- Keeper in the Hall 



adjustment has been substituted, which has the advantage of be- 

 ing applied without stopping the vibrations. 



If experience shall confirm the accuracy of Mr MITSCHER- 

 LICH'S experiment, and verify Dr BREWSTER'S inference from it, 

 an important advantage will have been gained by this applica- 

 tion, as a pendulum invariable in its own nature, must have a 

 great superiority over a compensation one, which, however well 

 adjusted to isochronism under differences of temperature, when 

 all its parts are affected simultaneously, must always be liable to 

 derangement from partial currents and changes. The small ex- 

 pense at which such a pendulum can be procured, would, in that 

 case, lead to making good time-keepers come into more general 

 use. 



The last peculiarity which I shall notice is of less import- 

 ance than those above mentioned, but, nevertheless, merits some 

 remark, as it tends to obviate another cause of irregular action in 

 the mechanism of time-keepers, viz. the gradual accumulation 

 of dust in the interior of the case. In order to understand 

 the utility of the contrivance which has been resorted to, to 

 prevent this, it is necessary to explain the way in which the ac- 

 cumulation of dust takes place. If a clock-case be closed up 

 when the air of the apartment is of a medium temperature, air 

 will be drawn into the case through the readiest apertures, as 

 soon as a diminished temperature causes a contraction of bulk in 

 the included air. The air which enters will carry with it a por- 

 tion of the dust which is always floating (as we see distinctly 

 when a sunbeam shines through a small aperture into a dark- 

 ened room). This dust is soon deposited, from the comparative 

 stillness of the air within the case, and when, by increase of tem- 

 perature, air is pressed out of the case, it leaves the dust behind 

 it ; by which means a small addition is made to the quantity of 

 dust in the case every time a contraction takes place, and thus, 

 in process of time, the action of the mechanism is impeded by 



