300 SECTION MECHANICS. 



which this specimen is a. frustum, in twenty-four hours. We can pro- 

 duce that model, nd in eight hours more we can test all its properties of 

 resistance. That is a rate of operation which would soon outrun our 

 powers of rationalised analysis, but we can attain it whenever an 

 occasion requires it. Without describing in detail the modus operandi 

 by which the model is produced, I may say that this material lends 

 itself very easily to all the usual foundry processes. We cast the 

 models in a mould made of clay, shaped approximately to the figure 

 required, by means of cross sections planted in due order in the clay, 

 so that we need rot produce a pattern model in the first instance. 

 The interior of the casting is kept hollow by a core, just as in the 

 common foundry process, but the core is made of a light frame 

 work, built in cross sections, covered with lath, and with a skin of 

 calico, which is covered with a thin coating of clay and plaister of Paris, 

 and thus rendered impermeable to the melted paraffin. The core, 

 slightly loaded with ballast, is placed in the mould, and rests in it 

 like a ship in a dock, leaving an intermediate space for the paraffin 

 between the core and the mould, and the paraffin is run into this 

 space. As the space is filled, water is poured into the interior of the 

 core, and thus prevents its floating up, and, at the same time, assists to 

 cool the paraffin ; and by the next morning the cold model may be 

 taken out, washed, and placed on the shaping machine. 



I will not describe that machine in full detail. It is simply a machine 

 in which the model travelling on a bed, like that of a planing machine, 

 between a pair of horizontal, highly speeded revolving-cutters which 

 possess a horizontal travel, transverse to that of the model, is operated 

 on by them at successive levels, their position being so governed that 

 they cut on the two sides of the model, a succession of true water 

 lines in pairs copied from the drawings of the model by an apparatus 

 under the control of an operator. Originally to guide the apparatus, 

 we used, instead of the water lines on the drawing, a succession of tem- 

 plates of the kind I have here, a very nice form, in which by the help 

 of adjustible steel ordinates, spring-steel ribbons are arranged in curves 

 to correspond with the waterlines in the drawing ; thus the series 

 of templates when fastened together in due superposition, would 

 constitute a skeleton model to the scale of the drawing ; but we have 

 lately found that, working with a magnifying glass, the operator can as 



