MECHANICS AND USEFUL ARTS. 19 



division of the circle was equal to 10 x 400 x 250= 1,000,000, --that 

 is, equal to an advance of the screw through a space of only one-mil- 

 lionth of an inch. This micrometer was placed at one end of a frame, 

 on which the bar to be measured was temporarily placed. When a 

 small piece of metal with its opposite surfaces parallel, and exquisite- 

 ly true, is placed between the bar and the micrometer, the latter is 

 screwed up until the small piece of metal (called the contact-piece) 

 is just nipped and held between them; then, if the screw be brought 

 back merely one-millionth of an inch, the contact-piece is loosened 

 and falls. Although the eye does not detect it, the machine does 

 veritably measure this infinitesimal quantity. As Sir Emerson 

 Tennent says, in his Story of the Guns : " So nice is the adjustment, 

 that, in using it, an inch of steel can be held to be an inch only so 

 long as the thermometer stands at 62 Fahrenheit, the slightest 

 excess of temperature producing an appreciable elongation. And the 

 standard yard, a square bar of steel, when placed in the machine, is 

 so expanded by the slightest touch of the finger, as to show an ap- 

 preciable lengthening even under the influence of the infinitesimal 

 amount of heat thus imparted ! Mr. Whitworth, like other inventors, 

 earns more honor than profit by such exquisite contrivances as these ; 

 but his profit begins when he applies the same principle in his work- 

 shop to produce articles in general demand. The result is seen in 

 many ways. Some years ago, there was a difficulty in working 

 metals to one-twentieth of an inch ; but the one-thousandth of an inch 

 is now worked as accurately as the one-twentieth was then." In 

 making the exquisite details of the Whitworth rifle, or in shaping and 

 adjusting the separate pieces, the workmen have come to regard the 

 ten-thousandth part of an inch (one-hundredth part of one-hundredth 

 of an inch) as a quantity within their cognizance, and on which their 

 credit as good workmen may depend. In the course of his elaborate 

 experiments on rifling, hexagonal bores, conoidal bullets, and so 

 forth, Mr. Whitworth made a cylinder 0*5770 inches, internal diam- 

 eter, a rod 0*5770 inches thick ; and another rod 0*5769 inches thick ; 

 the one rod fitted tightly into the cylinder when both were clean and 

 dry ; the other rod passed quite loosely into it ; and yet the two rods 

 differed in thickness only by the ten-thousandth part of an inch. 

 In short, "a little," to the last generation, would, by our present 

 mechanicians, be easily divided into a hundred parts. 



To produce minute results, instead of merely measuring them when 

 produced by others, is the purpose of many beautiful contrivances. 

 Photography is now one of the agents for effecting this. There are 

 little photographic pictures, not larger than a pin's head, containing 

 multitudes of portaits of distinguished persons ; a focalizing apparatus 

 produced them, and a microscope is necessary to render them 

 visible. 



At the great International Exhibition of 1862, many persons 

 glanced af the beautiful micrographical machine constructed by Mr. 

 Peters, the London banker; but the glance told little, except to 

 those who were previously somewhat versed in the subject. Most 

 attractive were the wonderful bits of writing on glass which had been 

 effected by the machine, and which could only be rendered visible by 

 the aid of powerful microscopes. Mr. Peters, as an amateur man of 



