The longitude of each canal was known, and the longitude of the central meridian of each drawing was always calculated and tabulated with the drawing, so that it was possible to tell which drawings might have shown the canal. Only when the position of the canal was within a certain number of degrees of the centre of the drawing (60°) was the drawing used in the result, allowance being duly made for the loss upon the phase side. Each drawing, it should be remembered, was as nearly an instantaneous picture of the disk as possible. It covered only a few minutes of observation, and was made practically as if the observer had never seen the planet before. In other words, the man was sunk in the manner. Such mental effacement is as vital to good observation as mental assertion is afterward to pregnant reasoning. For a man should be a machine in collecting his data, a mind in coördinating them. To reverse the process, as is sometimes done, is not conducive to science. When the successive true percentages of visibility of a given canal had thus been found, they were plotted vertically at points along a horizontal line corresponding in distance from the origin to the number of days after (or before) the summer solstice of the Martian northern hemisphere. The horizontal distance thus measured the time while the vertical height gave the relative visibility. The points so plotted were then joined by a smooth curve. This curve reproduced the continuous change in visibility undergone by the canal during the period under observation. It gave a graphic picture of the canal’s change of state. It seemed, therefore, proper to call it the canal’s cartouche or sign manual. In this manner were obtained the cartouches of 109 canals. Now, as the presence or absence of any canal in any drawing was entirely irrespective of the presence or absence of another, each such datum spoke only for itself, and was an entirely independent observation. The whole investigation thus rested on 10,900 completely separate determinations, each as unconditioned by the others as if it existed alone. As every factor outside of the canal itself which could affect the latter’s visibility was taken account of, and the correction due to it as nearly as possible applied before the cartouches were deduced, the latter represent the visibility of the canal _due to intrinsic change alone_. In other words, they give not the apparent only but the real history of the canal for the period concerned.