1840.] On the Construction and use of Portable Barometers. 261 



IV.— the Construction and use of Portable Barometers.— By Captain 

 J. Campbell, Assistant Surveyor General. 



1. — From some cause, not yet properly investigated, barometers, as 

 hitherto constructed fur portability, generally become damaged on the 

 voyage to this country, and even are liable to injury during transport 

 on land in this climate. I consider it likely that the plan on which 

 these instruments are constructed is faulty, and that the expansion of 

 the mercury in those, the tubes of which are closed by a screw, is suffi- 

 cient to cause the fracture of the tube. It is even probable, when 

 carried in an inverted position, that the expansion of the mercury in the 

 tube and the air in the cistern, which in this position will be of course 

 in the bottom part of it, is sufficient to force out the mercary through . 

 the pores of the cover, and that the alternate expansion and contraction 

 of this portion of air, according to the changes of temperature, may be 

 sufficient to force out all the mercury in the cistern, or to leave so little 

 in it, that, on setting up the instrument, there is not sufficient left to 

 cover the aperture of the tube. 



2. On examining a damaged mountain barometer, on " Newman's 

 improved portable iron cistern" plan, I consider I have found some rea- 

 son to suppose that the above remarks may be correct, and as the con- 

 struction of the instrument is not generally known, I append the fol- 

 lowing description. 



3. The cistern of this instrument is formed of a cylinder of iron 2| 

 inches long and 1 inch and three-tenths in diameter. The cover of it 

 is a close grained piece of box wood, cemented on to the tube and screw- 

 ed tightly into the cistern. Through the pores of this piece of wood 

 the air finds a passage to exert its pressure on the surface of the mer- 

 cury ; and, under considerable pressure, the mercury would probably also 

 be forced through the pores. The bottom of the cistern is double, the 

 lower part being about one-fifth of the capacity of the upper. This 

 lower part turns on a centre, by which it is attached firmly to the other 

 piece, and when turned to " not portable," as engraved on the brass 

 case, two holes in each part coincide, so as to open a passage between the 

 two divisions, and when turned to " portable" these passages are tight- 

 ly closed. In constructing the instrument, the tube having been 

 boiled, it is attached to the cistern by cement, the cistern is 

 then filled full of mercury, and the bottom part is then turned 

 to " portable", which closes the apertures of communication, 

 and leaves the part of the cistern with which the tube is 

 connected perfectly full of mercury. By a screw hole the mer- 

 cury ia the movable bottom is then allowed to run nearly all out, and, 



