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be not well and diligently considered, the sciences may indeed 

 be beautiful in theory, but are of no effect in practice. We call 

 the four instances referred to this class, mathematical instances 

 and instances of measure. 



Practice is laborious either from the multitude of instruments, 

 or the bulk of matter and substances requisite for any given 

 work. Thise 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. 



45. 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 in- 

 vestigated, not only to prevent deception, but to render practice 

 more extensive and efficient. For it is sometimes possible to ex- 

 tend 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 the 

 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 humors from above, and the 

 like. The magnetic power by which iron and the magnet, or 

 two magnets, are attracted together, acts within a definite and 

 narrow sphere, but if there be any magnetic power emanating 

 from the earth a little below its surface, and affecting the needle 

 in its polarity, it must act at a great distance. 



Again, if there be any magnetic force which acts by sympathy 

 between the globe of the earth and heavy bodies, or between 

 that of the moon and the waters of the sea (as seems most prob- 

 able from the particular floods and ebbs which occur twice in 

 the month), or between the starry sphere and the planets, by 



