266 On the Construction and use of Portable Barometers. [Oct. 



tube is softened, a little cone is easily pulled out. The end of this is to 

 be broken off, and the hole thus made opened and finished, to receive 

 the bent end of the long tube. The two tubes are then tied strongly 

 together with thread, and small pieces of cork between, and then the 

 junction between the tubes is to be made good with sealing wax, and 

 when cold, an inch of mercury being poured into the recurved end, the 

 tube is finished. Two tubes thus constructed, I have fuund by hourly 

 observations for many days, never to differ one-hundredth part of an 

 inch. 



11. But the best and most portable kind of barometer for India, is 

 with tubes to be filled wet, they can be carried empty, and therefore are 

 not liable to accident, and when wanted can be filled in five minutes. 

 The late Mr. James Prinsep, in the Journal of the Asiatic Society of 

 Bengal, has pointed out the advantage of this plan, and by frequent ex- 

 periment, and during long continued series of observations, I find tubes 

 thus filled are perfectly comparable together, and also with boiled tubes ; 

 the only precaution necessary is to make the column of mercury touch 

 ihs top of the tube by inclining the instrument about a minute before each 

 observation. To fill tubes thus, I use a little funnel of iron with a 

 small point and capillary tube attached to it. Having filled the tube 

 with mercury, it is to be set upright in some mercury in a cup, on which 

 half an inch of distilled or rain or any clean water has been poured. By 

 raising the end of the tube a little above the mercury, all the mercury 

 in the tube will descend, and its place be supplied with water. The 

 tube is to be then refilled with mercury, and again set up in the cup, 

 and again raised a little, so as to let one inch of water only ascend to 

 the top of the tube. A quantity of air bubbles extracted from the water 

 ■will ascend into the vacuum, and the column of mercury is to be oscil- 

 lated to assist their escape. The tube is then to be inclined until it is 

 filled, and the end being closed with the finger, it is to be removed from 

 the cup and inverted. The bubbles of air which had collected in the top 

 of the tube will now ascend to the open end, as also the water which 

 had been exposed in the vacuum. Mercury is then to be poured in 

 until the water lias reached the open end, and the tube is then to be set 

 Tip again in the mercury, and while upright the end is to be closed by the 

 finger, and inverted, by which the rest of the moisture in the bottom 

 of the tube will be exposed to the vacuum ; after which the tube is to 

 be again immersed in the mercury in the cup, the vacuum being still 

 preserved with the finger, which is then to be gradually removed. 

 The tube is now to be gradually and very gently inclined until 

 the water touches the top, and it is then to be removed and in- 



