BOOK II. INSTANCES OF THE ROD 529 



important instances. Practice is either deceptive or too laborious. 

 It is generally deceptive (especially after a diligent examination 

 of natures), on account of the power and actions of bodies being 

 ill defined and determined. Now the powers and actions of 

 bodies are defined and determined either by space or by time, or 

 by the quantity at a given period, or by the predominance of 

 energy ; and if these four circumstances be not well and dili 

 gently considered, the sciences may indeed be beautiful in theory, 

 but are of no effect in practice. &quot;\Vc call the four instances 

 referred to this class, mathematical instances and instances of 

 measure. 



Practice 13 laborious either from the multitude of instruments, 

 or the bulk of matter and substances requisite for any given 

 work. Those instances, therefore, are valuable, which either 

 direct practice to that which is of most consequence to mankind, 

 or lessen the number of instruments or of matter to be worked 

 upon. We assign to the three instances relating to this class, the 

 common name of propitious or benevolent instances. We will 

 now separately discuss these seven instances, and conclude with 

 them that part of our work which relates to the prerogative or 

 illustrious instances. 



XLV. In the twenty-first rank of prerogative instances we 

 will place the instances of the rod or rule, which we are also 

 wont to call the instances of completion or non ultra. For the 

 powers and motions of bodies do not act and take effect through 

 indefinite and accidental, but through limited and certain spaces; 

 and it is of great importance to practice that these should be 

 understood and noted in every nature which is investigated, not 

 only to prevent deception, but to render practice more extensive 

 and efficient. For it is sometimes possible to extend these 

 powers, and bring the distance, as it were, nearer, as in the 

 example of telescopes. 



Many powers act and take effect only by actual touch, as in 

 the percussion of bodies, where the one does not remove tho 

 other, unless the impelling touch the impelled body. External 

 applications in medicine, as ointment and plasters, do not exer 

 cise their efficacy except when in contact with the body. Lastly, 

 the objects of touch and taste only strike those senses when in 

 contact with their organs. 



Other powers act at a distance, though it be very small, of which 

 but few have as yet been noted, although there be more than 

 men suspect ; this happens (to take everyday instances) when 

 amber or jet attracts straws, bubbles dissolve bubbles, some 

 purgative medicines draw humours from above, and the like. 

 The magnetic power by which iron and the magnet, or two 

 Oiagneta, are attracted together, acts within a definite and nar- 



