P.
Penjal traps, 135.
Pepys and skating, 273.
Perry, coefficient of friction of greased surfaces, 265.
Phobos, dimensions of, 177; orbit of, 177.
Photoelectric activity and absorption, 207; persists at low
temperatures, 208, 209; not affected by solution, 213.
Photo-electric experiment, 205; sensitiveness of the hands, 207;
theory of latent image, 204, 209 _et seq._
Photographic reversal, experiments on, by Wood, 211; theory of,
210.
Piazzi, discovery of first Asteroid, 175.
Pickering, W. H., observations on Mars, 167.