270 On the Construction and use of Portable Barometers, [Oct. 



spurt out in radiating streams, all round the juncture with the cistern. 

 To remove the cistern, a small iron screw in the edge of it must be 

 sought for and removed, and it can then be screwed oif the cover. To 

 remove the cover of the cistern from the tube, it must be soaked for a 

 day or two in water at 140 deg. Farhenheit (hotter would break the 

 thermometer), which will soften the glue, and the tube and thermometer 

 must both be pulled out. The interior of the tube must then be washed 

 quite clean by filling it with distilled water, and emptying it again seve- 

 ral times. Generally a little dust of black oxide of mercury will be 

 found in it, which must be removed by dropping some nitric acid into 

 it, and washing out again with water. The tube is then to be filled wet 

 with mercury, and freed from air, as directed in para 11, and then 

 boiled. To cement the cover of the cistern on to the tube again, both 

 the tube and the cover being made hot, some sealing wax is to be ap- 

 plied around the tube, and the cover is to be twisted on, while the wax 

 is still soft. The cistern is to be cemented also on to the cover, by heat» 

 ing it strongly over a charcoal fire until it is hot enough to melt sealing 

 wax, and some wax softened in the candle being applied all round 

 inside the screw, the cistern being held by a thick cloth is to be quick- 

 ly screwed again on to the cover. When all is cold, the hole for the 

 thermometer is to be closed by a piece of soft wood, and the cistern 

 being filled with mercury through the two screw holes, the bottom is to 

 be turned, and the surplus mercury in the bottom is to be allowed to 

 run out. The screws having been inserted, the bottom is to be turned 

 again until the mark on the two pieces shews that the openings coin- 

 cide, and the tube is to be held upright. The next thing is to insert 

 the thermometer, which is now to be put into its place and cemented 

 with some putty made by rubbing chalk fine with wood oil, which will 

 dry in a couple of days. But as the thermometer is of no use, as it will 

 not shew the temperature of the mercury in the tube, where all the 

 hottest mercury of course ascends, it is better to remove it altogether, 

 and to stop the hole tightly with a deal peg. A thermometer attached 

 as in these instruments, is the more inconvenient, because very liable to 

 be broken, and then of course might endanger the whole instrument by 

 letting the mercury out of the cistern. To finish the barometer, noth- 

 ing more is afterwards necessary but to adjust to its old neutral point, 

 by comparison with a good barometer, by adding or removing a little 

 of the mercury in the cistern by the screw holes. 



