25 



length of the bar. (It has been found to be "02014554 inch.) 

 These contact pieces are not suited, however, for comparison of 

 bars with square or rectangular terminations, and others have 

 been prepared as shown in Plate IX., with small spherical 

 projections. They are rectangular, and are made of gun-metal 

 instead of steel. The distance between the lines in these studs 

 Las been determined to be '099232876 inch. By means of 

 these instruments measurements can be made accurately down 

 to the "00003187 of an inch, that being about the value of the 

 micrometer division. 



I need not detail the regulations to be observed for comparing 

 standards by this instrument ; it will suffice to say that most 

 definite and precise instructions are laid down, and the greatest 

 care is observed. The first observations are to be discarded, 

 and only the subsequent readings to be taken as authentic. 



The British Government, in rating the micrometer divisions, 

 used an inch measure of Messrs. Troughton & Simms' very 

 carefully divided into tenths and hundreds, together with a 

 scale of its real errors. The defining lines upon their scales 

 are cut upon silver, and like the lines in the gold studs of the 

 Exchequer standards, are so fine that it is only in the strongest 

 light that they are visible to the naked eye. To show the care 

 considered necessary by this department, the breadth of the 

 finest lines is nearly ten divisions of the micrometer, and the 

 human hair is about 100 divisions. 



The first work done by this instrument was the verifying 

 the Victorian Standard yard in 1866, which is similar to the 

 Imperial Standards, and is marked on sunk gold studs. I have 

 seen this standard in Melbourne, and am of opinion that it is 

 not a suitable form for these colonies. Under the Weights 

 and Measures Act, 1S85, which leaves the form of standard 

 yard optional, and in accordance with advice received lately 

 from Mr. Chaney, I am recommending a different form of yard 

 measure. 



The objection to the Victorian Standard is that to use it at 

 all a comparing instrument similar to that above described is 

 required, and this would be absurdly costty for us to employ ; 

 and again, there is no necessity for us to trouble about fourth 

 decimals of an inch. The allowance of error recommended 

 by the Commission in their report of 1841 was n ^j of the 

 whole, or '003 inch to the yard. This corresponds with a range 

 of temperature of ten degrees ; and the necessary accuracy 

 can be attained by means of much simpler standard known as 

 the Canadian Standard, and marked S.S., which I am recom- 

 mending, as mentioned above. 



Amongst the instruments used in comparing lengths are the 

 following: — An instrument for verifying measures of twenty 



