Mr. A. Bird on a Water-Barometer, 353 



in its place for some weeks, and four different times tap P was 

 closed, tap N opened, with tap Z, thus filling the barometer up 

 to tap L at top, which being opened allowed the water and 

 gaseous matter, if there had been any, to flow out at M. On 

 closing tap L and tap N and opening tap P, the column of 

 water again fell; and after siphoning out the surplus water 

 from under the oil in the cistern till the oil was level with the 

 zero-point E, the column of water was found on the four differ- 

 ent trials to be exactly the same height on the scale after each 

 trial as before. It was therefore plain that no gaseous matter 

 had accumulated above the water, and that, with the exception of 

 the vapour of water, it was a perfect vacuum. 



I will now mention one or two precautions which are required 

 in order to ensure success. In the first place the water must be 

 distilled — for this reason, amongst others, that if the water contains 

 " earthy salines " or colouring-matter, it is certain, by the con- 

 stant evaporation and precipitation in the working part of the 

 glass tube, to crust it over so completely, that in a few months 

 the water becomes invisible ; pure distilled water is therefore in- 

 dispensable. Then, if the slightest leak in the barometer exists, 

 it will infallibly bring the instrument to grief. In order, there- 

 fore, to be sure that the barometer was sound (before the water 

 deprived of air was put in), I closed tap L at top and tap P; 

 then, connecting the gutta-percha tube with the " street water- 

 works " pressure, I allowed it to enter the barometer till the in- 

 cluded air was contracted to one-fourth of its length, having a 

 pressure of water under it of between 40 and 50 lbs. to the inch. 



The barometer stood this internal pressure for ten hours 

 without the air being forced out. I therefore concluded that if 

 the barometer would stand this great pressure inside, it would 

 stand 14 lbs. to the inch pressure on the outside, and without 

 hesitation I filled it with the prepared water. 



As the instrument is made by a gas-fitter, it would be easy to 

 put the whole oj^it together, Lambert taps included, and to 

 prove it with some powerful water-pressure before the instru- 

 ment is taken to the place where it is to be erected. Also the 

 water deprived of air and covered with the stratum of olive oil 

 in the tin can could be sent, if necessary, 100 miles away with- 

 out the possibility of any air getting into it. If a gutta-percha 

 pipe is not to be had to fill the barometer, a piece of compo 

 tube will answer every purpose, which, when done with, is none 

 the worse for gas-fitting purposes. 



I shall conclude with some account of the action of the water- 

 barometer. In the Philosophical Transactions for 1832 is a de- 

 scription by Mr. Daniell of a water-barometer which he erected at 

 the " Royal Society's Rooms/' at Somerset House, which was in 



Phil. Mag. S. 4. Vol. 30. No. 204* Nov. 1865. 2 A 



