Page:Popular Science Monthly Volume 45.djvu/879

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the day; but if breaks formed in the clouds, the numbers began to rise, and the increase was very much in proportion to the amount of clear sky. It also appeared that these abnormal readings came more frequently with anticyclonic than with cyclonic circulation. The fact that during the days of abnormally high readings the air did not become hazed to anything like the extent indicated by the number of particles, seemed to suggest that these nuclei are of molecular dimensions, and it is even possible they may not be nuclei at all while the air is dry, and form nuclei in saturated air. The Kingairloch observations, when arranged in tables, showed that nearly double the number of particles are required to produce the same amount of haze when the air is very dry as when it is damp. The transparency of the air was also noticed to be roughly proportional to the wet bulb depression. It is not the amount of vapor in the air that produces this effect, but the nearness of the vapor to the dew point, which seems to enable the dust particles to condense more vapor by surface attraction and otherwise, whereby, by becoming larger, they have a greater hazing effect. In all densely inhabited areas the air loses its purity, and in all uninhabited areas it tends to regain it; but all uninhabited areas are not equally good purifying ones. Much of the dusty impurity discharged into our atmosphere from artificial sources, by volcanoes, and by the disintegration of meteoric matter, falls to the ground, but much of it is so fine it will hardly settle. The deposition of vapor on these small particles seems to be the method adopted by Nature for cleansing them away; they become centers of cloud particles and ultimately fall with the rain.

The Labors of a Woodpecker.—John B. Smith, of Rutgers College, New Jersey, writes to Garden and Forest that he has received a piece of white oak, thirteen inches in length and three inches in diameter, containing four holes made by a woodpecker. Each of the holes is nearly or quite an inch wide with the grain, and a trifle less across the grain, narrowing to the bottom of the holes; each of them reaches into the center of the tree and into an insect burrow. In order to reach one of the larva which were the object of its researches, the bird was compelled to make two attempts, having missed the point on the first attempt. The larva for which all this work was done measured about three quarters of an inch in length, with a diameter of perhaps one sixteenth of an inch, and would hardly serve to make more than a scant mouthful for even the smallest woodpecker. It must have taken the bird at least half an hour of persistent work to make each hole, or at least an hour to secure this one larva, weighing only a few grains. It seems, Mr. Smith remarks, as if it would be almost impossible to gain from such a larva a fair return in food value for the energy expended in getting at it, especially where it is necessary to make two efforts to recover one mouthful. In the other burrows the bird was more successful, and gained the larva at the first attempt.

A Forest in Nicaragua.—With the exception of a few clearings, the entire region of the San Juan River, Nicaragua, is described by B. Shimek, in his report to the Natural History Society of the State University of Iowa, as covered with typical tropical forests. They are almost impenetrable, except with the aid of the machete, with which the traveler must literally tunnel his way in many places through the walls of vegetation. The trees, many of which are very tall and from eight to fourteen feet in diameter, are not quite so closely placed together as those of our northern forests; but the intervening spaces are covered with shrubs and vines and numerous other plants, so that, particularly in lower places, dense jungles are formed. Moreover, each tree is a veritable garden in itself. The masses of parasites and epiphytes which cover the larger branches of the trees, and often extend down the trunk and along the smaller branches to their very tips, form a perfect canopy overhead through which the sun's rays never penetrate. Ferns, bromelias, orchids, mosses, and many other plants crowd their hosts with a dense mass of multicolored vegetation. In their active struggle for existence with more powerful neighbors of the forests, these plants have probably gradually ascended, in their search for the sun's light, to the upper branches of the very neighbors which sought to crowd them out, thus transferring the struggle from the surface of the soil: to the air above. So firmly