40 Notes on Barometer Construction, 



vial, treat these surfaces accordingly, and you will thereby 

 effect much towards the permanence of whatever instru- 

 ments you form from glass tubes. 



But there are two kinds of glass (chemically speaking) of 

 which barometer tubes are made; these may be dis- 

 tinguished in general terms as " crown glass" and '^ flint 

 glass" — I might say Continental glass and English glass, as 

 " crown" glass tubes prevail, as a manufacture, on the con- 

 tinent of Europe, while most of the English glass tube is 

 # of the " flint" variety. Besides the silicic acid and alkali 

 the crown glass contains a basis of lime, which is replaced 

 in the flint glass by lead oxide, so that "lime glass" and 

 " lead glass" are equally distinctive terms. The lead glass 

 is soft, the lime glass is hard ; the lead glass is easily fusible, 

 the lime glass is less easily fusible ; the lead glass has less 

 cohesive strength than the lime glass, as may be easily seen 

 by trying the breaking weights of rods or tubes (of equal 

 stoutness) of these two qualities of glass.* Lead glass is 

 more pellucid than lime glass ; tubes of the latter being 

 mostly striated throughout by lines which in reality are air 

 bubbles drawn into cylindrical cavities or threads of extreme 

 tenuity. Although the strength of lime glass may recom- 

 mend it for the construction of barometers to be used in 

 the field, on the other hand lead glass offers advantages for 

 instruments intended for indoor or laboratory use. The 

 lead glass is easier worked, is sufficiently strong for use in 

 careful hands, and in this material tubes free from defects 

 and of beautiful uniform transparency can be easily 

 obtained. 



Whatever the pattern of the barometer, the tube from 

 which it is to be made must be first examined as to equality 



* Experiment on cohesive strength of lead and lime glass tubes i-^ 

 Eelative weights of the glass tubes — 



A, lead glass ... ... 1123 grains 



B, lime „ 836 ,, 



Length of the tubes each 15 inches 



Bearing (wood) edges 10 inches apart 



Exterior diameter of each tube ... ... ... very nearly ^ inch 



Breaking weight of A ,. 32J lbs. avoir. 



,5 ,, B ... ... .t. ... 46 ,, „ 



Specific gravity of A 3*27 



„ B 2-509 



The tubes were gauged and selected so as to be as nearly as possible of 

 the same exterior diameter and diameter of bore ; the breaking weight 

 was gradually increased by progressive addition of lead bullets to a tared 

 BUBpended scale until fracture ensued. 



