HERO'S FOUNTAIN. 247 



it ceases entirely. This is due to the pressure of the air 

 enclosed in the flask ; while the water is driven out of 

 the tube, the air gradually expands, and the pressure 

 diminishes until it is again equal to the atmospheric pres- 

 sure; then no more water issues from the tube. The 

 washing bottle, fig. 36, differs from the apparatus just- 

 described in having an additional tube, which allows 

 air to be blown into it r while the jet of water issues from 

 the spout ; the jet may thus be maintained for a con- 

 siderable time. 

 1 



The apparatus may easily be put together by means of a piece of 

 tubing, drawn out at one end, a conk, and a, bottle which may, of 

 course, have a different shape from that in fig. 169 ; the tube should 

 be a few millimetres wide, but at the point only from O mm '5 to l mm . 

 The flask may be filled either by removing the cork, or by sucking 

 at the tube, and thus rarefying the air in the flask ; if the tube is 

 then closed and opened under water, the pressure of air outside 

 the flask is greater than inside, and water is driven into the flask 

 until the pressure of the air in it equals the atmospheric pressure. 

 If the quantity of water which has entered is not sufficient, the suck- 

 ing may be repeated, but the flask must now be held upside down, 

 for otherwise the end of the tube mignt dip into the water, and no 

 air, but water, would then be extracted from the flask. 



If the air in the flask is compressed by the pressure 

 of a column of water, the apparatus becomes a Hero's 

 Fountain. In fig. 170 A, there are two tubes, c and d, 

 one of which, , leads from the bottom of the flat dish 

 \ee on the top of the vessel a to the lower part of the 

 i vessel &, while the tube d leads from the top of b to 

 the upper part of a. A third tube,/, passes through 

 the flat dish e e to the lower part of the vessel a ; this 

 tube can either be taken out, as is the case in the 

 ornamental applications of this kind of fountain, 

 where the whole is usually fixed to the floor, and a 



