356 THE MATHEMATICAL PRINCIPLES [BoOK 11. Projectiles excite a motion in fluids as they pass through them, and this motion arises from the excess of the pressure of the fluid at the fore parts of the projectile above the pressure of the same at the hinder parts : and cannot be less in mediums infinitely fluid than it is in air, water, and quick silver, in proportion to the density of matter in each. Now this excess of pressure does, in proportion to its quantity, not only excite a motion in the fluid, but also acts upon the projectile so as to retard its motion ; and there fore the resistance in every fluid is as the motion excited by the projectile in the fluid ; and cannot be less in the most subtile aether in proportion to the density of that aether, than it is in air, water, and Quicksilver, in pro portion to the densities of those fluids. SECTION VIII. Of motion propagated through fluids. PROPOSITION XLI. THEOREM XXXII. A pressure is not propagated through a fluid in rectilinear directions unless ichere the particles of the fluid lie in a right line. If the particles a, b} c, d, e, lie in a right line, the pres sure may be indeed directly propagated from a to e ; but then the particle e will urge the obliquely posited parti te) cles / and g obliquely, and those particles / and g will not sustain this pressure, unless they be supported by the particles h and k lying beyond them ; but the particles that support them are also pressed by them ; and those particles cannot sustain that pressure, without being supported by, and pressing upon, those particles that lie still farther, as / and m, and so on in itiflnitum. There fore the pressure, as soon as it is propagated to particles that lie out of right lines, begins to deflect towards one hand and the other, and will be propagated obliquely in infinitum ; and after it has begun to be propagat ed obliquely, if 'it reaches more distant particles lying out of the right line, it will deflect again on each hand and this it will do as often as it lights on particles that do not lie exactly in a right line. Q.E.D. COR. If any part of a pressure, propagated through a fluid from a given point, be intercepted by any obstacle, the remaining part, which is not in tercepted, will deflect into the spaces behind the obstacle. This may be demonstrated also after the following manner. Let a pressure be propagat ed from the point A towards any part, and, if it be possible, in rectilinear