26 



feet to "01 inch, fitted with five microscopes and micrometers ; 

 calipers and micrometer for verifying measures of capacity ; an 

 apparatus for finding the expansion of metallic bars at varying 

 temperatures from 32° Fahr. to 212° Fahr. Small test-glass used 

 with micrometer microscopes, having minute subdivisions of 

 the millemetre cut upon it, known as Robert's lines. Nobert, 

 the German optician, has ruled bands containing 224,000 lines 

 to the inch ; he regularly makes plates with bands consisting 

 of from about 11,000 to 112,000 lines to the inch, numbered 

 first to nineteenth, which are used as microscopic tests. It is 

 of interest to note that the spaces between the lines on the 

 nineteenth band are about half the length of a wave of violet 

 light ; and the length of such a wave being "0000179 inch, and 

 the micrometer being capable of measuring to the half of one 

 division, viz., half of '0000318, it follows that the yard can be 

 measured to about the length of a wave of violet light.* 



The department is furnished with a complete set of Whit- 

 worth's plugs and diesinform as perillustration (Plate X.), vary- 

 ing from six inches diameter down to '1 of an inch by degrees of 

 '1 inch and "01 inch. In connection with these is a set of three 

 plugs and one die in a small case. The die is exactly one inch 

 in diameter ; one plug is exactly the same, but the other two 

 plugs are 1*001 inches and "999 inch diameter. The object of 

 this is to enable one to judge of an accurate fit. Upon applying 

 the plug one inch diameter I found that by keeping it moving, 

 and turning it as I pushed it, I could pass the plug into 

 the die and withdraw it ; but if I allowed it to stop for a 

 moment cohesion would take place, and some force was re- 

 quired to withdraw it. 



"When I applied the plug *999 inch in diameter, it easily 

 passed through, but without play, and without the experience 

 of the one-inch plug would have appeared a perfect fit. The 

 plug 1*001 inches in diameter would not pass in at all. These 

 are all made by Sir Joseph Whitworth & Co., and are examples 

 of perfect workmanship. 



Wire Gauges. — With reference to wire gauges, dissatisfaction 

 and uncertainty having previously existed, steps were taken 

 under certain clauses of the Weights and Measures of 1878 to 

 set this question at rest. At the Privy Council held on 23rd 

 August, 1883, an order was made to legalise certain wire 

 gauges, and establish them as standards under the Act ; and 

 although other gauges are in use, viz., a " Sheet and Hoop-iron 

 Gauge" (South Staffordshire Ironmasters' Association), Stub's 



* The periodical report by Board of Trade (of date February (5, 1882) 

 refers to the subdivisions of the inch to the 100,000th by means of micro- 

 meter screws. 



