222 PREPARATION FOR A NATURAL 



choice of the most important instances in each (such as 

 should be most abundantly and diligently searched and, as 

 it were, hunted out) must be governed by the prerogative 

 instances. 



vi. We must here allude to that which we have treated 

 more at length in the ninety-ninth, one hundred and nine 

 teenth, and one hundred and twentieth aphorisms of the first 

 book, and need now only briefly urge as a precept, namely, 

 that there be admitted into this history, 1 . the most common 

 matters, such as one might think it superfluous to insert 

 from their being so well known ; 2. base, illiberal, and filthy 

 matters (for to the pure every thing is pure, and if money 

 derived from urine be of good odour, much more so is know 

 ledge and information from any quarter), and also those 

 which are trifling and puerile ; lastly, such matters as ap 

 pear too minute, as being of themselves of no use. For (as 

 has been observed) the subjects to be treated of in this 

 history are not compiled on their own account, nor ought 

 their worth, therefore, to be measured by their intrinsic 

 value, but by their application to other points, and their 

 influence on philosophy. 



vn. We moreover recommend that all natural bodies 

 and qualities be, as far as is possible, reduced to number, 

 weight, measure, and precise definition ; for we are plan 

 ning actual results and not mere theory ; and it is a proper 

 combination of physics and mathematics that generates 

 practice. The exact return and distances of the planets, 

 therefore, in the history of the heavens, the circumference 

 of the earth, and the extent of its surface compared with 

 that of water, in the history of the earth and sea, the quan 

 tity of compression which the air will suffer without any 

 powerful resistance in the history of air, the quantity by 

 which one metal exceeds another in weight, in that of 

 metals, and a number of like points are to be thoroughly 

 investigated and detailed. When, however, the exact pro 

 portions cannot be obtained, recourse must be had to those 

 which are estimated or comparative. Thus, if we distrust 

 the calculations of astronomers as to distances, it may be 

 stated that the moon is within the shadow of the earth, and 

 Mercury above the moon, &c. If mean proportions cannot 

 be had, let extremes be taken, as that the feeblest magnet 

 can raise iron of such a weight compared with its own, 

 and the most powerful sixty times as much as its own 

 weight, which I have myself observed in a very small armed 

 magnet. For we know very well that determinate instances 



