THE BAROMETER. 



295 



in the lid of the teapot for the admission of air ; this hole serves the same 

 purpose as the hole for the vent-peg in the cask. 



Although it is not usually practised, a small hole should he made in the lid 

 of a kettle, but for a different reason. If the lid of a kettle fit it closely, so as 

 stop all communication between the external air and the interior of the vessel, 

 when the water contained in it becomes heated, steam will rise from its surface, 

 3rd the air enclosed in the space between the surface and the lid being heated, 

 wfll acquire an increased elastic force. From these causes, the pressure 

 which acts on the surface of the water in the kettle will continually increase 

 go long as the lid maintains its position ; this pressure, transmitted by the wa- 

 ter in the kettle, will overcome the pressure of the atmosphere acting on the 

 water in the spout, and the effect will be that the water will be raised in the 

 spout, and flow from it, or, if the lid be not firmly enough fixed to withstand 

 the pressure of the steam, it will be blown off the kettle. Such effects fall 

 within every one's experience. If a small hole were made in the lid these 

 effects would be prevented. 



Ink-bottles constructed so as to prevent the inconvenience of the ink thicken- 

 ing and drying, owe their efficacy to the atmospheric pressure. The quantity 

 of evaporation which takes place in the liquid, other circumstances being the 

 same, is proportional to the quantity of surface exposed to the external air. To 

 diminish this quantity of surface without inconveniently diminishing the quan- 

 tity of ink in the bottle, bottles have been constructed of the shape represented 

 in figure 8. 



A B is a close glass vessel, from the bottom of which a short tube, B, pro- 

 ceeds, from which another short tube rises perpendicularly. The depth of the 

 tube C is such as will be sufficient for the immersion of the pen. When ink 

 is poured in at C, the bottle, being placed in an inclined position, is gradually 

 filled up to the knob A : if the bottle be now placed in the position represented 

 in the figure, the chamber A B being filled with the liquid, the air will be ex- 

 cluded from it, and the pressure tending to force the ink upward in the short 

 tube C, will be equal to the weight of the column of ink, the height of which 

 is equal to the depth of the ink in the bottle A B, and the base of which is 

 equal to the section of the tube C. This will be manifest from the proper- 

 ties of hydrostatic pressure, established in hydrostatics. Now, the atmo- 

 spheric pressure acts on the surface C with a force which would be capable 

 of sustaining a column of ink many times the height of the bottle A B ; conse- 

 sequently, thrs pressure will effectually resist the escape of the ink from the 

 mouth C, and will keep it suspended in the bottle A B. In this case the 

 whole surface which is exposed to the effect of evaporation, is the surface of 

 liquid in the tube C, and, consequently, an ink bottle of this kind may be left 

 many months in a warm room and no perceptible diminution in the quantity of 

 ink or change in its quality will take place. As the ink in the short tube C is 

 consumed by use, its surface will fall to a level with the tube B. A small 



