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ANNUAL REPORT SMITHSONIAN INSTITUTION, 1910. 



steam has expelled the air and is issuing through the narrow tube 

 the latter is sealed off. When the whole has cooled it will be found 

 that the J tube acts as a water hammer, i. e., if, by inclining the tube 

 the water is made to travel from end to end, its concussion makes a 

 metallic ring. This is owing to the fact that very little air has been 

 included when the tube was sealed, and water vapor at normal tem- 

 peratures is unable to act as an elastic pad in the same way as air at 

 normal atmospheric pressure would. The clicking metallic ring then 

 may be taken as an indication that the gas pressure within the tube 

 is very slight. Care must indeed be taken not to let the concussion' 

 become too violent, as in that way the tube may be easily shattered. 



If now, by carefully inclining the tube, the long limb is com- 

 pletely filled with water (fig. 2 B) and all the bubbles are chased 

 out of that limb b} r holding the bent end uppermost, so that no breaks,' 

 even the most minute, remain, we shall find, on inverting the tube 



and bringing the bent end under, that the 

 Avater remains in the long limb and does 

 not under the force of gravity take up the 

 lowest possible level in both limbs (fig. 

 2 C). From the level in the two limbs it 

 is evident that the hydrostatic pressure of 

 the shorter column can not possibly balance 

 the pressure of the column in the longer 

 limb. The one is about 85 centimeters 

 higher than the other. The water in this 

 case, like the sulphuric acid in Donny's 

 experiment, hangs in the tube. The liquid 

 in the long limb is in contact with the glass 

 all over, and, since it wets it perfectly, it 

 adheres to it. To the film of water adhering to the glass the rest 

 of the water coheres, and this cohesion is much more than able to 

 sustain the weight of the column of water which is counterbalanced 

 by no other upholding force. In this way the lower part of the 

 water in the longer limb of the tube transmits through the upper 

 part a stress to the glass equivalent to its gravitational pull. 



The reality of this pull becomes all the more striking when, by 

 destroying the cohesion at one spot, a rupture is started. This rup- 

 ture, which may be at first invisibly small, rapidly spreads across the 

 whole column. The rupture may usually be started by a sharp knock 

 administered to the side of the longer limb; but, when the cohesion 

 is very perfect, to produce a rupture may require a shock so violent 

 as to be liable to shatter the tube. When the rupture is started, the 

 lower part tears suddenly away from the upper part of the column 

 and falls into the bend of the tube. The upper part follows it more 

 slowly, trickling down the inside of the tube, and all the water comes 

 to occupy a position in the lower part of the tube (fig. 2 A). 



Fig. 



