III. TRUE SURFACES AND ACCURATE MEASUREMENTS. BY D. 

 W. ROBB, A. S. M. K, Amherst, N. S. 



(Received 24th November, 1895). 



That absolute truth is almost unattainable, becomes apparent 

 to the skillful mechanic as well as to the thoughtful scientist, 

 and the degree of success of each may be measured by the 

 nearness of approach to absolute accuracy, whether it be in the 

 result of the scientist's reasoning, or the more material product 

 of the mechanic's hands. 



For the production of flat surfaces of metal, the mechanic 

 uses an instrument called the " surface plate," which is simply a 

 plate of cast iron well stiffened by ribs and resting upon three 

 points of support to prevent springing, the upper side of which 

 is carefully scraped by hand until its surface is approximately 

 true. When one of a pair of such " surface plates " is placed 

 with its trued surface above the other it will not immediately 

 come in contact with the lower plate, but will for a time float 

 upon the air confined between the surfaces, because the air can 

 only escape at the edges, and, as the plates come closer together, 

 it will do so more slowly so that a noticeable time, depending 

 upon the truth of the surfaces, weight and size of plates, will 

 elapse before the plates will really touch each other. When the 

 air is fully excluded, or as fully as the truth of the surfaces will 

 allow, the plates will adhere, or rather the atmospheric pressure 

 on the outside will press them together. 



It is evident that, if we interpose between the surface plates 

 a fluid, more viscid than air, such as oil, it will require a longer 

 time and greater weight to expel it, and we may move the 

 upper plate back and forth for a long time before the plates 

 come into direct contact, the particles of fluid forming a perfect 

 system of rollers, upon which the metal will roll with very 

 slight friction. If we provide means whereby the oil will be 

 renewed, the iron will never come into contact, provided the 



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