126 



Laboratory Arts 



III. PLASTER CASTING. 



The main usefulness of this art is in the preparation of 

 models, of certain specimen duplicates, and to provide moulds 

 in which may be cast lead weights suitable for pendulum, lever, 

 and other experiments. 



As an example of the first, we may take the manufacture 

 of a composite cube consisting of six square based pyramids. 



Set out on stiff drawing paper or thin white cardboard, a 

 square on a 2" base, and construct the development of a square 

 based pyramid, the height of which will be i". The correct 



FIG. no. Development of square based pyramid. 



.N.B. By an oversight a flange upon the second side of the last face has been 

 omitted in the above sketch. 



length of the edges may be found geometrically by joining a 

 point i" from A along the diagonal AB to either of the ends 

 of the other diagonal of the square. With B as centre, and 

 this distance as radius, cut AC, the bisector of the angle at A, 

 at the point D, which is the centre of a circle on the circum- 

 ference of which the bases of the remaining three faces are set 

 off. Join to D and add all necessary flanges. 



With a sharp knife score all the edges, and bend into 

 shape, touching the flanges with good glue, and attaching them 

 on the inside of the pyramid. 



