said to be able to subsist on fruits and berries in winter, and to be so active upon the trees as to catch small birds among the branches. So also the woodchuck of Canada has a dark-brown fur; but then it lives in burrows and frequents river banks, catching fish and small animals that live in or near the water.

Among birds, the ptarmigan is a fine example of protective coloring. Its summer plumage so exactly harmonizes with the lichen-colored stones among which it delights to sit, that a person may walk through a flock of them without seeing a single bird; while in winter its white plumage is an almost equal protection. The snow-bunting, the jerfalcon, and the snowy owl are also white-colored birds inhabiting the arctic regions, and there can be little doubt that their coloring is to some extent protective.

Nocturnal animals supply us with equally good illustrations. Mice, rats, bats, and moles possess the least conspicuous of hues, and must be quite invisible at times when any light color would be instantly seen. Owls and goatsuckers are of those dark mottled tints that will assimilate with bark and lichen, and thus protect them during the day, and at the same time be inconspicuous in the dusk.

It is only in the tropics, among forests which never lose their foliage, that we find whole groups of birds whose chief color is green. The parrots are the most striking example, but we have also a group of green pigeons in the East; and the barbets, leaf-thrushes, bee-eaters, whiteeyes, turacos, and several smaller groups, have so much green in their plumage as to tend greatly to conceal them among the foliage.

SOIL WASTAGE THROUGH TILLAGE 1

As soon as agriculture begins, the ancient order of the soils is subverted. In order to give his domesticated plants 1 From N. S. Shaler's Man and the Earth.

a chance to grow, the soil-tiller has to break up the ancient protective mantle of plants, which through ages of natural selection became adjusted to their task, and to expose the ground to the destructive action of the rain. How great this is may be judged by inspecting any newly plowed field after a heavy rain. If the surface has been smoothed by the roller, we may note that where a potsherd or a flat pebble has protected the soil it rests on top of a little column of earth, the surrounding material having been washed away to the streams, where it flows onward to the sea. A single heavy rainstorm may lower the surface of a tilled field to the amount of an inch, a greater waste than would, on the average, be brought about in natural conditions in four or five centuries. The result is that in any valley in which the soils are subjected to an ordinary destructive tillage the deportation of the material goes on far more rapidly than their restoration by the decay of the underlying rocks. Except for the alluvial plains whereupon the flood waters lay down the waste of fields of the upper country, nearly all parts of the arable lands which have been long subjected to the plow are thinned so that they retain only a part of their original foodyielding capacity. Moreover, the process of cropping takes away the soluble minerals more rapidly than they are prepared, so that there is a double waste in body and in the chemical materials needed by the food-giving plants.

There is no question that the wasting of soils under the usual tillage conditions constitutes a very menacing evil. Whoever will go, with his eyes open to the matter, about the lands bordering on the Mediterranean, will see almost everywhere the result of this process. Besides the general pauperizing of the soils, he will find great areas where the fields have prevailingly steep slopes from which the rains have stripped away the coating down to the bedrock. In

Italy, Greece, and Spain, this damage has gone so far that the food-producing capacity of those countries has been greatly reduced since they were first subjected to general tillage. There is no basis for an accurate reckoning, but it seems likely from several local estimates that the average loss of tillage value of the region about the Mediterranean exceeds one-third of what it was originally. In sundry parts of the United States, especially in the hilly country of Virginia and Kentucky, the depth and fertility of the soil has in about one hundred and fifty years been shorn away in like great measure. Except in a few regions, as in England and Belgium, where the declivities are prevailingly gentle, it may be said that the tilled land of the world exhibits a steadfast reduction in those features which give it value to man. Even when the substance of the soil remains in unimpaired thickness, as in the so-called prairie lands of the Mississippi valley, the progressive decrease on the average returns to cropping shows that the impoverishment is steadfastly going on.

THE FORMATION OF VAPOR DROPS 1

The formation of the vapor drops that make the cloud was long a puzzle to science, but modern research has at last succeeded in solving this mystery. It has been found by experiments that if pure dry air, and pure vapor of water, be mixed in a clean vessel, and then cooled down below the temperature of saturation, the drops of mist are not generally able to form. Purity means that all the particles of dust which float in the air have been perfectly filtered out, and that all traces of electricity have been removed from the air before mixing them. It was further discovered that if fine dust powder is injected into the 1From an article by Frank H. Bigelow on "The Formation and Motion of Clouds," Popular Science Monthly, April, 1902.

pure mixture, without changing the temperature or the pressure, the drops of water developed at once; also that if minute charges of electricity, carried on particles of matter which may be as small as one-thousandth part of the mass of an atom of hydrogen, are introduced, the drops are able to condense. It is inferred that nuclei of some kind, dust particles or electric particles, called ions or electrons, are required for the formation of water drops suspended in dry air, one nucleus for each drop. Hence, it is possible, by counting the number of minute drops that form in a cubic inch, to estimate the number of motes of dust in the air, and even the number of ions charged with electricity in a given volume. The number of the ions contained in the air may be enormous, ranging from 20 per cubic centimeter to many millions. We perceive further that these minute drops coalesce to form rain, which falls from the clouds to the ground, and that they carry down the dust previously blown up by the winds and so purify the atmosphere from all sorts of small floating particles. They also bring electric charges to the earth, and this has something to do with producing the atmospheric electrical potential which always exists. These ions are a natural portion of the atmosphere itself, being continuously produced in it, even when no special cause seems to be present, and they have much to do with explaining some of the strange characteristics of atmospheric electricity which have so long baffled all efforts to comprehend. Investigators are now paying the closest attention to these ions from every point of view.

A SAILOR'S WORK 1

Nothing is more common than to hear people say—" Are not sailors very idle at sea?-what can they find to do?"

1 From Two Years Before the Mast, by R. H. Dana, Jr.

This is a very natural mistake, and being very frequently made, it is one which every sailor feels interested in having corrected. In the first place, then, the discipline of the ship requires every man to be at work upon something when he is on deck, except at night and on Sundays. Except at these times, you will never see a man, on board a well-ordered vessel, standing idle on deck, sitting down, or leaning over the side. It is the officers' duty to keep every one at work, even if there is nothing to be done but to scrape the rust from the chain cables. In no state prison are the convicts more regularly set to work, and more closely watched. No conversation is allowed among the crew at their duty, and though they frequently do talk when aloft, or when near one another, yet they always stop when an officer is nigh.

With regard to the work upon which the men are put, it is a matter which probably would not be understood by one who has not been at sea. When I first left port, and found that we were kept regularly employed for a week or two, I supposed that we were getting the vessel into sea trim and that it would soon be over, and we should have nothing to do but to sail the ship; but I found that it continued so for two years, and at the end of the two years there was as much to be done as ever. As has often been said, a ship is like a lady's watch, always out of repair. When first leaving port, studding-sail gear is to be rove, all the running rigging to be examined, that which is unfit for use to be got down, and new rigging rove in its place: then the standing rigging is to be overhauled, replaced, and repaired, in a thousand different ways; and wherever any of the numberless ropes or the yards are chafing or wearing upon it, there "chafing gear," as it is called, must be put on. This chafing gear consists of worming, parceling, roundings, battens, and service of all kinds-both ropeyarns, spun-yarn, marline, and seizing-stuffs. Taking