96 Domestic Science 



weighing in air due to the volumes of air displaced by 

 both the objects weighed and the weights used. 



58. We live in a gaseous atmosphere and as a 

 result are under pressure due to the weight of that 

 atmosphere every moment of our lives. That we. are 

 not ordinarily conscious of this pressure is explained 

 by the fact that our bodily structure is specially adapted 

 for existence under the average fluid pressure of the 

 air. The troubles of balloonists and mountain-climbers 

 at high altitudes, as well as those of divers in a diving- 

 bell and workers in the caissons used in tunnelling, are 

 caused by the great differences of air-pressure that they 

 are called upon to experience. The magnitude of the 

 pressure of the air is generally measured by methods 

 illustrated in the succeeding experiments. 



EXPERIMENT 33. Obtain three glass tubes, closed 

 at one end, and about 36" in length. Fill the first with 

 water, put the forefinger over the open end, and invert 

 the tube with its end under the surface of some water 

 in a beaker or other suitable vessel. Remove the finger 

 and clamp the tube in an upright position with its open 

 end still under the water-surface. Similarly fill a 

 second tube with glycerine and invert it in a vessel 

 containing that liquid. Fill the third tube with 

 mercury and invert it in a shallow bowl of mercury. It 

 will be noticed that the tubes containing water and 

 glycerine remain completely filled with liquid, while 

 in the case of the mercury the tube is only partly filled 

 with that liquid, the surface of the mercury in the tube 

 standing about 30 inches higher than the level of the 

 liquid in the outer vessel. 



It is evident that the weight of the liquid in each 

 tube tends to cause the liquid to fall out of the tube 

 into the vessel below. The force which counterbalances 



