not be difficult to prove the same thing, with regard to those he has assigned to the earth. If perchance he has been right in ascertaining the qualities belonging to fire, (although in our _Physical Paradoxes_ we have denied his position with respect to its being hot in the highest degree) I say, if he has been right in this particular, it was not because philosophy had penetrated the secret, but because the thing was manifested to us by our senses. XI. These are the organs, by which all the natural truths we are able to obtain a knowledge of are conveyed to our spirits. Even in the mathematical faculties, which affect to confide every thing to theoretical demonstrations, they cannot, except in the two elemental ones, arithmetic and geometry, safely advance a step, without holding before them, the light of experience. It was this taught the geographer, the situation and position of the divers parts of the globe; the navigator, the directive virtue of the load-stone; the static philosopher, the weight, the descent, and acceleration of motion in their descent, of heavy bodies; the mechanic and engineer, the augmentation of power by machinery, or the multiplication of purchase; the astronomer, the movements and course of the stars; the musician, the consonant and dissonant intervals of music; the optician, perspective, or the effects of vision when the eye surveys distant objects; to the catoptrician, and dioptrician, the laws of reflexion, and refraction. SECT. IV. XII. And it is very worthy of remarking, that even after experience had made those first discoveries, on which the theory and practice of arts are founded, those primitive lights were in general found to be insufficient to enable the understanding to make further advances in them, but it was rather found to be necessary, that the same experience should continue as they proceeded on, to direct their steps, and correct their errors. I will explain what I mean, by two examples taken from navigation.