‹‹‹ Vitr. 10.16.4 | Table of Contents | Vitr. 10.16.6 ›››
5For the same principles do not answer in all cases. In some machines the principles are of equal effect on a large and on a small scale; others cannot be judged of by models. Some there are whose effects in models seem to approach the truth, but vanish when executed on a larger scale, as we have just seen. With an auger, a hole of half an inch, of an inch, or even an inch and a half, may be easily bored; but by the same instrument it would be impossible to bore one of a palm in diameter; and no one would think of attempting in this way to bore one of half a foot, or larger.
5For not all things are practicable on identical principles, but there are some things which, when enlarged in imitation of small models, are effective, others cannot have models, but are constructed independently of them, while there are some which appear feasible in models, but when they have begun to increase in size are impracticable, as we can observe in the following instance. A half inch, inch, or inch and a half hole is bored with an auger, but if we should wish, in the same manner, to bore a hole a quarter of a foot in breadth, it is impracticable, while one of half a foot or more seems not even conceivable.