620 



REPORT — 1898. 



(3) By placing the oi'iginal curve above the other in a series of con- 

 secutive positions, and in each position drawing round the outline with a 

 pencil or pen. 



Fig. 1. 



This latter method proved to be by far the most satisfactory, and 

 fig. 2 gives one or two examples of the mode in which the outline of the 

 required envelope is thus produced. 



Via. 2. 



Fig. 3 is a more compact form of the instrument, in which, in order to 

 obtain circles of different diameters, a sliding centre was used. 



In these cases there is a certain amount of inconvenience from the 

 axle of the upper pair of edge runners, and although the frame was 

 inclined, yet the axle itself prevented the curve being conveniently drawn, 

 and the arrangement, shown in tigs. 4 and 5, indicate the method by 

 which the necessary driving shaft could be kept entirely beneath the sur- 



