C. II. Warren — A Graduated Sphere. 495 



circles, a pencil may be held firmly against one of the vertical 

 scales while the sphere is rotated about its axis by means of 

 the cup in which it rests, or a small metal clip could be easily 

 made which would serve to hold the pencil instead of using 

 the lingers. 



The accuracy which can be attained with this sphere de- 

 pends, of course, on the accuracy of its construction. A skill- 

 ful mechanician should, however, be able, with a little pains, 

 to construct it 'so accurately that the results obtained with it 

 will be of the same order of accuracy as those which can be 

 obtained by the use of stereographic plats, provided, of 

 course, that the same amount of care is taken in drawing, and 

 in reading the angles. It has seemed to the writer that this 

 form of a graduated sphere has an advantage over that of 

 Nikitin in being somewhat more flexible and easier to use. It 

 has been found to be invaluable for purposes of rapid demon- 

 stration in the laboratory. 



The cost of the sphere built for the writer was about forty 

 dollars (labor and materials). The standard, however, was 

 taken from another piece of apparatus, so that the probable 

 cost of the sphere and mountings would be in the neighbor- 

 hood of fifty dollars. 



Department of Geology, Massachusetts 



Institute of Technology, 



Cambridge, Mass., Aug. 1916. 



