Proceedings. 185 



rate measurement of wires. Mr. JOHNSON exhibited the dif- 

 ferent forms of gauge down to the screw gauge. His own 

 new gauge is an adaptation of the micrometer screw which, 

 for certain practical purposes, is handier than Sir Joseph 

 Whitworth's, and was made under Mr. JOHNSON'S in- 

 structions. It consists of two short pillars, in one of 

 which a stud is fixed, and on the other a horizontal screw 

 with a pitch "020" works. A barrel about 2" diameter is 

 mounted on the screw ; the circumference of the barrel is 

 very carefully divided, first into divisions which represent 

 "OO 1 " and then again into divisions representing 'oooi". The 

 smallest divisions are about sV apart. The article to 

 be measured is placed between the fixed stud and the 

 end of the horizontal screw and the measurement read 

 off on the barrel. As the divisions reading -oooi" are - 3 V 

 apart, a difference of even 30V0" can be easily detected. 

 Mr. JOHNSON illustrated the delicacy of the instrument by 

 reading off the increase in length of a small piece of metal 

 held for a few seconds over a lamp. An animated discussion 

 on the reliability of measurements of more minute differ- 

 ences, with special reference to Sir Joseph Whitworth's 

 machine, ensued, Mr. JOHNSON expressing scepticism. — 

 Dr. Williamson asked how it is possible to make 

 mechanical arrangements so perfect as to measure quantities 

 which it is only possible to see under the highest powers of 

 the microscope, as for instance in the case of a diffraction 

 grating? — Mr. Gwyther pointed out that the diffraction 

 gratings are themselves cut by the aid of mechanical appa- 

 ratus of similar construction. — The President remarked 

 that, as Sir Joseph Whitworth maintained, the accuracy of 

 using all instruments of the kind depends upon the cultiva- 

 tion of a delicate sense of touch, and that, since Whitworth, 

 no one has professed to use his instruments with the 

 accuracy to which he had trained himself. 



Dr. Alexander Hodgkinson read a paper on 



