366 . REPORT — 1866. 



the same liberal and enlightened spirit, and in conjunction with the many 

 able and excellent men, in both Houses of ParUamout, whether in or out of 

 office, who now combine their eflbrts in the same direction. The extensive 

 exhibition and use of our Mural Standard will, as we may confidently anti- 

 cinate, be among the principal means of accomplishing the object. 



"The Committee of the British Association, soon after its appointment, 

 thought it desirable to appply for advice and assistance to the Chemical 

 Society, which includes many of the most eminent chemists and metallur- 

 gists in the kingdom. Their application was granted in the kindest manner 

 by the President of the Society, Dr. William Allen Miller, the Secretary, 

 Dr. Odling, and the other members of the Council. The subject was brought 

 before the Society at two of its meetings, and the result was a very important 

 change in the course of proceeding. Professor Frankland advised that, 

 instead of Baily's metal, or any other metallic substance, either simple or 

 compound, the Mural Standard should be made of white glazed porcelain. 

 The question was carefully considered, more especially in regard to the 

 durability of porcelain, and its susceptibility of changes by expansion and 

 contraction. With regard to durability, we know from innumerable exam- 

 })lcs that porcelain Avill last for hundreds of years without any perceptible 

 decay. We also know that it is very little subject to expansion and con- 

 traction from the changes of atmospheric temperature. But it is also well 

 known that all objects made of clay contract by exposure to great heat. 

 How could we pass our porcelain Standard through the ordeal of a fur- 

 nace without destroying the dimensions marked upon it? In this diffi- 

 culty we were fortunate in obtaining the assistance of Mr. Casella, Philoso- 

 phical Instrument Maker to the Board of Ordnance. This gentleman, whose 

 business makes him familiar with works of this particular description, instituted 

 a series of experiments, which proved that a slab of porcelain after completion 

 contracts visibly on its reexposure to a great heat, but that, if the heat be 

 sufficiently intense and sufficiently long continued, an adequate security will 

 be obtained against future change. Consequently a slab may be prepared by 

 firing at fii'st, and then have the lines etched in with hydrofluoric acid, the 

 figures and letters painted with enamel, the lines rubbed in with the same, 

 and then the lines, figures, and letters all burnt in, after which treatment it 

 will not shrink at all. Having obtained so satisfactory a result, the Com- 

 mittee desired Mr. Casella to proceed with his work. 



The Committee have seen no reason to make any important change in the 

 form and dimensions of the instrument. These remain nearly as they were 

 sho^vn to the Statistical Section of the British Association at Birmingham. But, 

 as the Yard was then placed in close contact with the Metro, a question arose 

 whether the two measures might not be more clearly distinguished from 

 each other ; and to effect this it was proposed that the Yard should be marked 

 in red lines and the Metre in blue. This suggestion was adopted, and the 

 instrument, thus completed, is thought to be elegant and attractive as well 

 as clear and distinct. If, however, any ]x>rsons prefer having it marked 

 with black lines this may be done. 



It was requisite that the divisions should be so exact that no inaccuracy 

 could be perceived either by the sight or the touch. This has been accom- 

 j)hshed by our artist, who obtained from M. Perreaux, of Paris, one of his 

 beautiful dividing instruments, which is so constructed as to divide, if le- 

 quired, to the 500th part of a millimetre, a length far more diminutive than 

 can ever be found necessary. About the tenth of a miUimetrc is sufficient 

 to answer every useful purpose. 



I 



