In the many papers which have been published on the measurements of volume, density, and porosity of pebbles or rock specimens, relative accuracy cannot be reasonably compared from one paper to another. Hence, a rather thorough set of experiments was undertaken in order to compare volume measurements by two widely accepted methods, namely, mercury immersion and water immersion. Fifty pebbles, as non-porous as readily available and not exceeding 32 mm in diameter, were selected; and 750 volume measurements were made. The standard deviation, coefficient of correlation, coefficient of determination, and standard error of estimate revealed useful facts concerning the closeness of association in the two methods. Of the pebble volumes determined by mercury, 99.837 percent are closely related to the volumes determined by water.