Page:Electricity (1912) Kapp.djvu/24

20 The knowledge of this coefficient enables us to determine for any two bodies the attractive force if their masses, configuration and relative position are given. For spheres the calculation is quite simple, but for bodies of more complicated shape it is very difficult, and sometimes only possible in rough approximation. It would, for instance, hardly be possible to accurately calculate the mass attraction between two Dreadnoughts lying side by side, but by using the general formula and the coefficient $$f$$ as here determined we get as a rough approximation a force of $$7$$ lb.

It is, of course, out of the question to check such a calculation by direct experiment, since disturbing causes, such as the slightest breath of wind striking the side of the ship, will produce a disturbing force many times greater than the force to be measured. If, however, we could eliminate all disturbing forces, then a direct determination of $$f$$, quite independent of astronomical observation, would be possible. Such determinations have been made by Cavendish, Maskelyne, Airy and others, the most recent being Poynting's, carried out in the Birmingham University. Professor Poynting has measured, by means of an exceedingly delicate balance, the attractive