i840.] On the Construction and use of Portable Barometers. 265 



9. Daniel has pointed out that many barometers are made and sold 

 in London, the tubes of which are not boiled, and the divisions are laid 

 ojffby comparison with other instruments. This is also often fraudulent- 

 ly done with instruments for exportation, and when they are made 

 with close cisterns, this fraud is difficult to detect, but in syphon baro- 

 meters is easily discovered, because the mercury in an unboiled tube 

 will not stand within an inch nearly of its proper altitude, and if the 

 divisions have been laid off from the ends of the mercurial column instead 

 of from a central point, the distance between the divisions, as numbered 

 on the top and bottom parts of the instrument, will not be found to 

 agree. Thus between 7 on the lower part, and 7 on the upper part, 

 should be 14 inches exactly ; if it is less, then the instrument has been 

 improperly made. 



10. Those who have sufficient mechanical skill and ingenuity to boil 

 the tubes themselves, may provide themselves with instruments like the 

 syphons above described at a much lower price by ordering the brass 

 cases without tubes, and a stock of empty lubes to be sent ; for which 

 Messrs. Jones charge only 8 shillings each. These tubes are sent out in 

 two pieces ; the recurved part being separated, and to be attached at the 

 lamp. But the method by which I prefer to fit them up, is the following. 

 I bend the end of the long part at right angles, and then cut it off close 

 to the side, so that a piece of 2 or 3 tenths of an inch projects ; I then 

 fill the tube with water, and then with mercury, and free the upper part 

 from air, as hereafter described. I then heat the closed end gradually 

 over a chauffer, and make all the moisture simmer up to the open end, 

 carrying the tube gradually across the fire. The closed end is then again 

 heated (the tube being held at an angle of 20 degrees with the horizon), 

 until the mercury first becomes speckled and then forms large bubbles, 

 "which by making the tubes revolve, are carried gradually and spirally 

 towards the open end, collecting all the smaller bubbles in their way. 

 The mercury must never be allowed to boil and snap, for if this is al- 

 lowed, minute portions of gas, hardly visible when cold, are carried down 

 towards the closed end, and cannot be removed except as directed. The 

 gas I consider to be probably hydrogen, evolved from the decomposition 

 of minute organic substances in the tube. I find by frequent experi- 

 ment that filling the tube first with water is the best method to get all 

 the air out, instead of carefully drying the tube as generally directed. 

 The tube being properly boiled, the part for the recurved end is to be 

 sealed at the lamp, and the end finished as customary by blowing, and 

 then the rounded end of a rod of glass being applied to the side of the 

 tube, I an inch from the sealed end, and this being pulled when the 



