Laboratory at St. John's College, Oxford. 181 



condensed in the ventilating-tubes, and injured the machinery. 

 The arrangement has been removed. 



The principal work done since that time has been the con- 

 struction of two disk machines and the clock governor. One 

 rather elaborate disk machine with clock governor attached is 

 finished and ready for experiment. The disks are made of 

 brass, and have thirty radial slots. If the machine is suc- 

 cessful, it will enable determinations of tuning-forks to be 

 made with great accuracy by merely striking them and looking 

 at them through the revolving disk. 



A good deal of incidental work has been done, on the prin- 

 ciple that it is best to provide complete and efficient apparatus 

 for every point as it arises. In particular, considerable addi- 

 tions have been made to the lathe, including an overhead 

 motion (the existing back-motion proving insufficient for 

 some purposes). 



In slotting the disks in the lathe, it was found that saws of 

 the ordinary form were unsuitable for the purpose, as they 

 soon choked when imbedded in the brass. This difficulty 

 was overcome according to a principle suggested by Willis. 

 Where choking takes place, the number of cutting-teeth is to 

 be reduced. A small steel circular saw was therefore con- 

 structed with four teeth only on the circumference ; by means 

 of this the disks were slotted on the lathe without difficulty. 

 A new tool, however, has been recently invented in Oxford, 

 which promises to overcome this difficulty by the application 

 of more power to the ordinary saw. It will also extend greatly 

 the applicability to all forms of circular cutter, whose use has 

 hitherto been limited by the same difficulty. 



The first disk machine constructed consisted simply of a 

 solid foundation carrying the spindle on which the disk re- 

 volved, with a small driving-pulley, and a counting-arrange- 

 ment. It was driven by a fine string w T hich passed round the 

 driving-pulley and a large pulley on the shafting over head. 

 The string slipped without much friction on the driving-pulley. 

 So long, then, as the speed of the shafting was greater than that 

 corresponding to the required speed of the disk, the disk could 

 readily be kept to any exact speed by the application of any 

 simple form of break. I think this arrangement very pro- 

 mising ; but, from various circumstances, it did not seem to be 

 exactly what I was immediately in want of, and I proceeded 

 to construct the second machine, which is of a more elaborate 

 character. 



The driving part of the second machine is worked by an 

 endless rope and a weight. The rope passes also through an 

 independent machine by which the winding-up is effected 



