Construction of Wottcustoif* Goniometer. 21 5 



any error in dividing or centering the graduated limb corrected ; 

 this addition, however, I have not found it necessary to adopt. 



As it may be useful to those members who have turning- 

 lathes, to know how these may be converted into accurate angu- 

 lar instruments, I shall detail the necessary modifications. 



The first requisite is a slow motion or tangent screw for the 

 spindle. That usually adopted in the theodolite answers very 

 well ; still better would be that employed by Robinson in his 

 small astronomical circles. No turner of any standing can feel 

 the least difficulty in constructing this for himself. The next 

 thing is to obtain an accurately graduated circle. For this a 

 circle of brass should be accurately fitted to some part of the 

 spindle or pulley, and made so carefully that it may be re- 

 moved and replaced with certainty. This must be sent to the 

 dividing engine to receive the graduations. The limb on my 

 lathe is 6.9 inches in diameter, and is graduated in thirds of de- 

 grees ; the subdivision into single minutes being effected by 

 means of a vernier. As soon however as possible, I mean to 

 construct a micrometer for the same purpose, because by its 

 help the position of the eye in reading off' may be rendered 

 more convenient ; and also because the edge to which the ver- 

 nier applies has become considerably rounded. 



From this addition, the lathe receives a great augmentation 

 of power. It can be employed as a graduating instrument, 

 particularly in cases where the graduations are to be unequal, 

 as in logarithmic circles, &c. ; or it is competent to the construc- 

 tion of orrery wheels, when the deviation from the mean mo- 

 tions are to be shewn ; it is also useful, in conjunction with the 

 graduations of the slide rest, for placing a series of points on a 

 piece of work by means of their polar co-ordinates. I find no 

 difficulty in guiding my tool by such means, within the thou- 

 sandth part of an inch of the required place. 



To use the turning lathe thus fitted up as a reflecting gonio- 

 meter, we have only to provide an adjusting chuck which may 

 enable us to adjust the position of the crystal attached to it. 

 The plan I adopted is this. Being already in possession of a 

 cylindric chuck, with eight adjusting screws, which I used for 

 turning pivots accurately, I took a square bar of iron and 



