442 An Easy Method of [Dec. 



at the other end. This quantity will remain in, if the tuhe 

 is held at but slight declivity. The wide end being now 

 closed with the finger, the pouring tube is to be removed 

 to the barometer tube, which should be held mouth up, at 

 an inclination of about 45°. The point of the pouring tube 

 being entered into the mouth, the finger is to be withdrawn, 

 and the mercury poured in, by increasing the declivity of 

 the pouring tube. Thus the mercury runs down to the 

 closed end, and the air passed up by the horse hair, leaving 

 few or no bubbles. When it contains 3 inches of mercury, 

 however, it should be carefully examined all round, and if 

 any bubbles appear, they should be brought to the hair, by 

 gently tapping the tube, held almost horizontal with a bit 

 of wood, at the same time turning it slowly a little back- 

 ward and forward upon its axis, the hair being never 

 allowed to go below. This should be done at every 3 or 4 

 inches, to have a smooth column of mercury as the filling 

 proceeds. When the tube is thus full, the hair is to be 

 withdrawn, leaving an end of it in the vacancy left by its 

 removal, until that also is filled. The hair being now 

 withdrawn altogether, the tube is to be overfilled, so that 

 the mercury presents a convex face above the glass. 



The open end is now to be stopped with a finger, just 

 moistened to give it closeness; which squeezing out the 

 superfluous mercury will effectually prevent all access of 

 air. The tube is now to be inserted in the mortar of quick- 

 silver, and brought to a vertical position, when a vacuum 

 will be produced by the descent of the mercury. 



The lower end is now to be again tightly closed with the 

 finger, the tube lifted out of the mortar and brought gently 

 to a horizontal position. The finger must be kept tight 

 against the open end, to maintain the vacuum ; when a 

 minute portion of air will make a visible bubble in any part 

 of the column. By lowering the head a very little, the 

 mercury may be made to flow gently to that end, and leave 

 the vacuum next to the finger. By a short jerking motion 

 in the direction of its length, the tube and mercury are 

 kept in a sort of vibration, the mercury striking smartly 

 against the closed end, like the water hammer, and this 

 vibration brings together and carries upward toward the 

 finger, any bubbles which may be present in the column. 



