THE HOSE-LEVEL 169 



6 feet from the end held to the ground, lift the hose to 

 the height of 5 feet above the ground, hold it in that posi- 

 tion for fifteen seconds, and then, holding the hand shoulder 

 high, move it very slowly under the hose to the farther 

 end. Any bubbles of air which may have been held by 

 the water in the hose should follow the upper bend in 

 the hose produced by the hand passing under it, and should 

 be liberated through the tube at the farther end. The 

 end near the ground may be lifted any time after enough 

 hose has passed so that some portion shall touch the ground 

 between the end and the hand moving the air bubbles. 

 It is well to keep the thumb held tightly over the end of 

 the tube until the hand reaches the farther end. Exercise 

 care not to remove the thumb until the tubes are brought 

 to the same height. Water will probably have to be added 

 now to bring up the water in the tubes. If added slowly, 

 no air need be introduced below the tubes. 



220. Checking up the instrument. After introducing 

 the water and removing any air present, the two ends of 

 the apparatus should be brought together, side by side, 

 against some vertical surface upon which is drawn a 

 horizontal line or which may have a horizontal upper edge, 

 approximately shoulder high. Raise or lower one of the 

 ends until the top of the water column stands even with 

 the line or edge. The column at the other end should also 

 stand even with the line or straight edge. Repeat the 

 test after disturbing the main portion of the hose by 

 lifting it at different points or partly or wholly stretch- 

 ing out a part or all of it. If the columns stand at the 

 same height in each of three trials, it may be put into 

 use at once. 



If in any trial the columns fail to stand at the same 

 height, this indicates the presence of air in the hose ; the 



