Biography of Percival Lowell

by A. Lawrence Lowell

Strangely enough fine lines, from the continuity of the impression they make upon the eye, can be recognized when of a thickness that would be invisible in the case of a mere dot. To determine how narrow a line on Mars would be perceptible, experiments were made with a wire of a certain size, noting the limit of distance at which it could be seen; and then, from the magnifying power of the telescope, it was found that a Martian canal would be visible down to about a mile wide. From this the conclusion was drawn that the canals probably ran from two or three up to fifteen or twenty miles in width, the minimum being much less than had been thought at earlier oppositions. The distance apart of the two branches of double canals he estimated at about seventy-five to one hundred and eighty miles, save in one case where, if a true instance of doubling, it is over four hundred. Of the oases, whereof one hundred and eighty-six had been observed, much the larger part were from seventy-five to one hundred miles in diameter.

The later oppositions enabled him also to complete the topography of the planet, showing that the canals were a vast system, running from the borders of both polar caps, through the dark areas of natural vegetation where they connected, at obviously convenient points, with a still more complex network in the ochre, or desert, regions, and thus across the equator into the corresponding system in the other hemisphere. By this network the greater part of the canals could receive water alternately from the melting of the north and south polar caps, or twice yearly, the Martian year, however, being almost twice as long as our own. But to perfect his proof that this actually takes place he had to show that the canals, that is the streaks of vegetation bordering waterways, sprang into life—thereby becoming visible or darker—in succession as the water spread from the poles to the tropics; and this he did with his usual thoroughness at the opposition of 1903.

Since there was then no mechanical means of measuring the variations in visibility of the canals,—and under the atmospheric conditions at any place in the world perhaps there never will be,—the record had to be made by the eye, that is in drawings by the observer as he saw the canals; and these, as he said, must be numerous, consecutive and extended in time. The consecutive could not be perfectly carried out because “as Mars takes about forty minutes longer to turn than the Earth, such confronting (of the observer) occurs later and later each night by about forty minutes, until finally it does not occur at all while Mars is suitably above the horizon; then the feature passes from sight to remain hidden till the difference of the rotations brings it round into view again. There are thus times when a given region is visible, times when it is not, and these succeed each other in from five to six weeks, and are called presentations. For about a fortnight at each presentation a region is centrally enough placed to be well seen; for the rest of the period either ill-placed or on the other side of the planet.” But with changes as gradual and continuous as those of the darkening of the canals this did not prove a serious drawback to the continuity of the record.

There was another element in the problem. The drawing being the estimate of the observer on the comparative darkness of the markings from time to time it was of the greatest importance to avoid any variation in personal estimates, and therefore Percival made all the drawings himself. From April 6 to May 26 he drew the planet every twenty-four hours, and although “the rest of the time did not equal this perfection, no great gap occurred, and one hundred and forty-three nights were utilized in all.... But even this does not give an idea of the mass of the data. For by the method employed about 100 drawings were used in the case of each canal, and as 109 canals were examined this gave 10,900 separate determinations upon which the ultimate result depended.”

For each canal he plotted the curve of its diminishing or increasing visibility as the season advanced, and this curve he called the cartouche of the canal. Now combining the cartouches of all the canals in each zone of latitude, he found that those in the several zones began to become more distinct—that is the vegetation began to come to life—in a regular and approximately uniform succession, taking from the northern arctic down to the equator and past it to the southern sub-tropic about eighty Martian days. From north latitude 72° to the equator, a distance of 2,650 miles, took fifty-two of these days, at a speed of fifty-one miles a day, or 2.1 miles an hour. Now all this is precisely the opposite of what happens on the Earth, where vegetation in the spring starts in the part of the temperate zone nearest to the equator, and as the season advances travels toward the pole; the reason for the difference being, he says, that what is needed on Earth to make the sap run is the warmth of the sun, what is needed on Mars is water that comes from the melting of the polar snows. He points out also that the water cannot flow through the canals by nature, because on the surface of a planet in equilibrium gravity would not draw it in any direction toward or away from the equator. “No natural force propels it, and the inference is forthright and inevitable that it is artificially helped to its end. There seems to be no escape from this deduction.” In short, since water certainly cannot flow by gravity both ways in the same canal, the inhabitants of Mars have not only dug the canals, but pump the water through them.

OBSERVING AND DRAWING THE CANALS OF MARS

In recapitulating the reasons for the artificial character of the canals he shows a most natural annoyance with people who doubted the validity of his observations; and, in dealing with the evidence to be drawn from the fact that they run on great circles, that is on the shortest lines from one point to another, he writes: “For it is the geodetic precision which the lines exhibit that instantly stamps them to consciousness as artificial. The inference is so forthright as to be shared by those who have not seen them to the extent of instant denial of their objectivity. Drawings of them look too strange to be true. So scepticism imputes to the draftsman their artificial fashioning, not realizing that by so doing it bears unconscious witness to their character. For in order to disprove the deduction it is driven to deny the fact. Now the fact can look after itself and will be recognized in time.”

This last prophecy was largely verified before these three oppositions of the planet came to an end. In 1901 photography was tried without success so far as the canals were concerned. For the stars it had worked very well, for to quote again: “Far less sensitive than the retina the dry plate has one advantage over its rival,—its action is cumulative. The eye sees all it can in the twentieth of a second; after that its perception, instead of increasing, is dulled, and no amount of application will result in adding more. With the dry plate it is the reverse. Time works for, not against it. Within limits, themselves long, light affects it throughout the period it stands exposed and, roughly speaking, in direct ratio to the time elapsed. Thus the camera is able to record stars no human eye has ever caught and to register the structure of nebulae the eye tries to resolve in vain.