Lectures de Potentia Restitutiva, or of Spring

Having thus explained the most simple way of springing original: "springing"; here meaning elasticity or the "spring" of a material in solid bodies, it will be very easy to explain the compound way of springing—that is, by flexure bending—supposing only two of these lines joined together as at G H I K.

When these are bent by any external power into the form L M N O, the line L M will be extended, and N O will be diminished in proportion to the flexure. Consequently, the same proportions and rules for its endeavor to restore itself the force of elasticity returning the object to its original shape will hold true.

In the next place, we consider fluid bodies. The greatest instance we have of these is air, though the same principles exist in some proportion in all other fluid bodies.

The air, then, is a body consisting of particles so small as to be almost equal to the particles of the heterogeneous fluid medium the "ether" or atmospheric gases encompassing the earth. It is bounded only on one side—namely, towards the earth—and is indefinitely extended upward. It is only hindered from flying away in that direction by its own gravity (the cause of which I shall explain at some other time). It consists of the same particles, single and separated, of which water and other fluids are composed when conjoined and compounded. Being made of particles exceeding...