27 



iron-wire gauge, and steel-wire gauges, no other but the 

 Imperial Standard gauge is strictly legal. 



Expansion by Heat. — General Baeyer some time ago pointed 

 out a phenomenon with regard to the expansion of metallic 

 bars by heat. He found upon careful verification of certain 

 measuring bars used in measuring an arc of meridian that the 

 co-efficient of expansion had sensibly decreased, while the 

 length of the bars at their normal temperature had during 

 twenty years remained the same. To explain this, he ventures 

 three hypotheses — 1st. That at each time the bars are subjected 

 to increased temperature a step is made towards reducing the 

 temper of the steel, and thus rendering it less expansive. 

 2nd. That a crystalline structure is set up by the vibrations 

 and concussions these bars were subject to. 3rd. A somewhat 

 complicated hypothesis, based on the fact that certain sub- 

 stances exist consisting of two sets of wholly different crystal 

 systems, due to high and low temperatures, and that the bars 

 remaining at ordinary temperature, there is a tendency to 

 reduce the system of crystals to a third, corresponding to the 

 medium temperature. He admits himself that this is least 

 probable. 



WEIGHTS. 



The Imperial Weights and Measures Act, and Acts derived 

 from this, specify the standard pound to be of platinum, and 

 the weighing to be conducted in vacuo. The object of this will 

 be evident — and is, of course, the elimination of the disturbing 

 effect of specific gravity, and platinum being the densest of all 

 metals is least influenced by the buoyancy of the air. As a 

 matter of fact the comparisons are not usually made in vacuo, 

 though provision is made for this, but careful observations of 

 the barometer and thermometer are made and corrections applied 

 — based upon the ascertained specific gravity of the compared 

 and comparing weights. Indeed, to such a pitch of refinement 

 and delicacy are verifications of important standards carried 

 that the expansion of the metal due to temperature is observed 

 and corrections are made for this. Professor Schumacher in 

 his paper on " The Comparison of the late Imperial Standard 

 Pound "Weight with a Platina Copy of the Same and with 

 Other Standards of Authority," read before the Royal Society 

 in 1836, says : — " It is to be hoped that no pound will in future 

 ever be declared a legal standard unless its specific gravity 

 and expansion (the knowledge of which is indispensable, even 

 for a single comparison with a good balance) are previously 

 determined with the greatest possible precision." 



As it has been decided that immersion in water is calculated 

 to injure the standards, their density must be determined by 



