224 Notices respecting Neic Books. 



good workman becomes familiar with such sizes as -001 and -0001 

 of an inch, he wall not rest satisfied until he can work with corre- 

 sponding accuracy " (pp. 73, 74). The instrument whereby the 

 workman is enabled to become practically acquainted with these 

 minute divisions, is the workshop measuring-machine ; the primary 

 object of which is the construction of accurate difference-gauges, 

 e. g. of a series of solid cylinders whose diameters shall form an 

 arithmetical progression having a common difference of l-5U00th 

 of an inch. It consists essentially of two small planes accurately 

 parallel to each other, and capable of being adjusted at any assigned 

 distance by means of screws with accurately divided heads. The 

 article to be measured (say a solid cylinder) is placed between these 

 planes, and one of them is moved forward until the cylinder 

 experiences in passing between them a slight resistance, the in- 

 tensity of which can be judged of by the touch ; the distance be- 

 tween the planes can then be determined by reading the screw- 

 heads. As the planes touch the cylinder, their distance is its 

 diameter. The machine resembles a small turning-lathe : it consists 

 of a cast-iron bed and two head-stocks, one fixed, the other capable 

 of being moved along the bed by a double-threaded screw of a half- 

 inch pitch. By this screw the movable head-stock can be quickly 

 placed in an approximately right position. The measuring-planes 

 are the ends of cyliudi'ical pieces exactly fitted into cylindrical 

 holes in the head-stocks ; they are caused to advance and recede 

 by screws with 20 threads to the inch — the head of the screw in 

 the movable head-stock being divided into 250, that in the fixed 

 head-stock into 500 equal parts. There are therefore three gra- 

 duated parts, viz. a scale of inches on the bed, a screw-head reading 

 to l-5000th of an inch in the movable head-stock (A), and a screw- 

 head reading to 1-lOOOOth of an inch in the fixed head-stock (B). 

 Suppose it is required to produce a gauge 4-003 inches in diameter. 

 The screw-head B is set and read ; the movable head- stock is 

 brought up to the four-inch graduation on the bed, and clamped ; 

 the screw-head A is then turned till a 4-inch standard gauge just 

 passes freely between the planes, which are now four inches apart ; 

 the wheel B is then set back 30 divisions, making the planes 4-003 

 inches apart ; and the piece of metal is gradually formed till it has 

 perfect but free contact with the planes. When the true adjust- 

 ment has been given to the machine, it is found that an increase or 

 diminution of the distance between the planes corresponding to a 

 quarter of a di^■ision of the large scre\^-head, and amounting there- 

 fore to l-40000th of an inch, causes a distinctly perceptible dimi- 

 nution or increase in the resistance which the object encounters in 

 passing between them, and consequently the error in the diameter 

 of the gauge ought not to exceed the 40000th part of an inch. 



"When the object in A'iew is to produce an exact copy of a standard 

 of length, c. g. of a standard inch, a machine called " the millionth- 

 measuring machine " is used. This is constructed on a similar 

 plan to the workshop machine, above described ; but now the head- 

 stocks are both fixed, and the measuring-planes are wrought on the 



