S8 Notes on Barometer Gonstrtiction. 



If we consider the mode of manufacture of these glass 

 barometer tubes we shall easily understand their liability to 

 the conical as distinguished from the cylindrical form. A 

 hollow stout cylinder of soft semi-molten glass is formed on 

 the end of the blowing tube, and a second heated blowing 

 tube is attached to the outer end of the ductile mass. The 

 two workmen, each holding one of these blowing irons, 

 retreat from each other until the glass tube is drawn down 

 to the requisite diameter, say until they are twenty or thirty 

 feet or more apart. A ladder of suitable length has been 

 laid on the floor, and on this the glass tube is now laid and 

 detached from the blowing rods at each end. It is even- 

 tually cut into six-feet or three-feet lengths, in which state 

 the tube is ready for removal to the anneahng lear (if it be 

 annealed at all). The " butts," that is to say, the two outer 

 lengths which were in immediate contact with the blowing 

 irons^ are sensibly conical, and the other segments of the 

 entire tube are liable in degree, according to their position, 

 to this conicity, and therefore it is a point of primary 

 importance to gauge the tubes during selection in the 

 manner already described, so as to obtain pieces which are 

 sensibly cylindrical. 



There are certain other points in selecting the glass tubes 

 which will require attention — clearness of the glasSj freedom 

 from knots, and similar defects, &c.; but these are too 

 obvious to require further mention. 



As barometer tubes are required in most cases to be of 

 stout glass, it therefore becomes necessary that they should 

 have been effectually annealed ; and here enters into the 

 consideration a curious point of interest. I think I need 

 not hesitate to say that much of the glass tube met with in 

 commerce is either imperfectly annealed, or^ as in the case 

 of tubes with thin walls, it has not been annealed at all. 

 The question of the degree of annealing which each kind of 

 tube requires is regarded I believe in a purely commercial 

 spirit ; providing what will sell, and especially regarding 

 the consideration of cheapness of production. As there is 

 more in this statement than might catch our attention, I 

 ask your patience while I go into the question a little more 

 fully. Unannealed glass is glass in a condition of strain or 

 unequal tension, and that portion of it which is unduly 

 stretched is liable, on slight prompting, to rupture ; such 

 glass will not bear sudden vicissitudes of temperature, or 



