THE BAROMETEE. 



237 



is, the vertical distance between the tops of the 

 mercury in both branches of the tube, remains un- 

 affected by the action of capillarity. It will be evident 

 that in a siphon-barometer any change in the atmo- 

 spheric pressure affects the mercury in both branches 



A B 



90- 



80 



70- 



60- 



50 



40- 



30- 



20- 



10 



FIG. 164 (i real size}. FIG. 165 



imultaneously; hence the scale is either moveable and 

 allows of the zero-point being placed on a level with the 

 lower surface of the mercury, as shown in fig. 1 64 A ; 

 or the zero -point is placed somewhere between the two 



