[10] Not all gravitational fields may be transformed away by a proper choice of coordinates. If this were so, the space, whose nature is independent of any choice of coordinates, would always be Euclidean.--Author. [11] Thus when it it said that a body contracts or that a clock runs slow when it is put in motion no actual physical change is implied. The judgment of different observers--one at rest with respect to the body and one not--are different.--Author. [12] The balance of Dr. Royds' essay is given to a discussion of the phenomena of Mercury's perihelial advance, the deflection of light under the gravitational field of the sun, and the shift in spectral lines, in connection with which alone Einstein's theory makes predictions which are sufficiently at variance with those of Newtonian science to be of value in checking up the theory observationally. In the interest of space conservation and in the presence of Dr. Pickering's very complete discussion of these matters we omit Dr. Royds' statement.--Editor. [13] There is, on any view, no difference as regards observation of position only.--Author. End of the Project Gutenberg EBook of Einstein's Theories of Relativity and Gravitation, by J. Malcolm Bird