moisture in the morning. If the temperature of the external air be at or below the freezing point, this deposition will form a rough coating of ice on the pane. Let a small piece of tin-foil be fixed on a part of the exterior surface of one pane of the window in the evening, and let another piece of tin-foil be fixed on a part of the interior surface of another pane. In the morning it will be found that that part of the interior surface which is opposite to the external foil will be nearly free from ice, while every other part of the same pane will be thickly covered with it. On the contrary, it will be found that the surface of the internal tin-foil will be more thickly covered with ice than any other part of the glass. These effects are easily explained by the principle of radiation. When the tin-foil is placed on the exterior surface, it reflects the heat which strikes on the exterior surface, and protects that part of the glass which is covered from its action. The heat radiated from the objects in the room striking on the surface of the glass, penetrates it, and encountering the tin-foil attached to the exterior surface, is reflected by it through the dimensions of the glass, and its escape into the exterior atmosphere is intercepted; the portion of the glass, therefore, covered by the tin-foil, is in this case subject to the action of the heat radiated from the chamber, but protected from the action of the external heat. The temperature of that part of the glass is therefore less depressed by the effects of the external atmosphere than the temperature of those parts which are not covered by the tin-foil. Now, glass being, as will appear hereafter, a bad conductor of heat, the temperature of that part opposite to the tin-foil does not immediately affect the remainder of the pane, and consequently we find that while the remainder of the interior surface of the pane is thickly covered with ice, the portion opposite the tin-foil is comparatively free from it. On the contrary, when the tin-foil is placed on the internal surface, it reflects powerfully the heat radiated from the objects in the room, while it admits through the dimensions of the glass, the heat proceeding from the external atmosphere. The portion of the glass, therefore, covered by the tin-foil, becomes colder than any other part of the pane, and the tin-foil itself receives the same temperature, which is not reduced by the effect of the radiation of objects in the room, because the tin-foil itself is a good reflector of heat, and a bad absorber. Hence the tin-foil presents a colder surface to the atmosphere of the room than any other part of the surface of the pane, and consequently receives a more abundant deposition of ice.

If a body, which is a good radiator of heat, be exposed in a situation where other good radiators are not present, it will have a tendency to fall in its temperature below the temperature of the surrounding medium; because, in this case, while it loses heat by its own radiation, its absorbing power is not satisfied by a corresponding supply of heat from other objects. A clear sky, in the absence of the sun, has scarcely any sensible radiation of heat; if, therefore, a good radiator be exposed to the aspect of an unclouded firmament at night, it will lose heat considerably by its own radiation, and will receive no corresponding portion from the radiation of the firmament to repair this loss, and its temperature consequently will fall.

A curious experiment made by Dufay affords a striking illustration of this fact. He exposed a glass cup, placed in a silver basin, to the atmosphere during a cold night, and he found in the morning a copious deposition of moisture on the glass, while the silver vessel remained perfectly dry. He next reversed the experiment, and exposed a silver cup in a glass basin. The result was the same: the glass was still covered with moisture, and the metal free from it. Now metal is a bad radiator of heat, and consequently has a tendency to preserve its temperature. Glass is a much better radiator, and has therefore a tendency to lose its temperature. These vessels being exposed to the aspect

of a clear sky, received no considerable rays of heat to supply the loss sustained by their radiation. This loss in the metal was inconsiderable, and therefore it maintained its temperature nearly or altogether equal to that of the air; the glass, however, radiating more abundantly, and absorbing little, suffers a depression of temperature. The glass, therefore, presented a cold surface to the air contiguous to it, and reduced the temperature of that air, until it attained that temperature at which it was below a state of saturation with respect to the vapor with which it was charged; a deposition of vapor, therefore, took place on the glass.

This discovery of Dufay remained a barren fact until the attention of Dr. Wells was directed to the subject. The result of his inquiries was the discovery of the cause of the phenomena of dew, and affords one of the most beautiful instances of inductive reasoning which any part of the history of physical discovery has presented. Dr. Wells argued that, as a clear and cloudless sky radiates little or no heat toward the surface of the earth, all objects placed on the surface which are good radiators must necessarily fall in temperature during the night, if they be in a situation in which they are not exposed to the radiation of other objects in their neighborhood. Grass and other products of vegetation are, in general, good radiators of heat. The vegetation which cov ers the surface of the ground in an open, champaign country, on a clear night, will therefore undergo a depression of temperature, because it will absorb less heat than it radiates. This fact was ascertained by direct experiment, both by Dr. Wells and Mr. Six. A thermometer, laid on a grass plot on a clear night, was observed to sink even so much as 20° below another thermometer suspended at some height above the ground. The vegetables, which thus acquire a lower temperature than the atmosphere, reduce the air immediately contiguous to them to a temperature below saturation, and a proportionally copious condensation of vapor takes place, and a deposition of dew is formed on the leaves and flowers of all vegetables. In fact, every object, in proportion as it is a good radiator, receives a deposition of moisture. On the other hand, objects which are bad radiators are observed to be free from it. Blades of grass sustain large, pellucid dew-drops, while the naked soil in their neighborhood is free from them.

In the close and sheltered streets of cities the deposition of dew is very rarely observed, because there the objects are necessarily exposed to each other's radiation, and an interchange of heat takes place which maintains them at a temperature uniform with that of the air. A deposition of dew, in this case, can only take place when the natural temperature of the air falls below its point of saturation.

In an obscure, cloudy night no deposition of dew takes place, because in this case, although the vegetable productions radiate heat as powerfully as before, yet the clouds are also radiators, and they transmit heat, which, being absorbed by the vegetables, their temperature is prevented from sinking much below that of the atmosphere.

METEORIC STONES & SHOOTING STARS.

Indactive Method.-Appearances accompanying Meteorites.-Theories to explain them.-Examination of these Theories.-Shooting Stars.-November and August Meteors.-Orbits and Distances.— Heights-Chladni's Hypothesis.

METEORIC STONES & SHOOTING STARS.

WHEN We reflect upon the length of time which has elapsed since just methods of investigating nature were first formally taught by BACON, we can not fail to be struck with surprise at witnessing the frequency with which those inestimable precepts are neglected and overlooked. There appears to be a disposition inherent in the mind-springing probably from that arrogance and vanity, which are invariably the offspring of ignorance-that induces a disposition, in every case, precipitately to rush to the formation of theories and the assumption of causes, omitting, or postponing, the far more important though less ambitious duty of analyzing phenomena. It is true that these observations are less applicable to that order of minds which have been disciplined in the severe schools of the old and long-established universities, where the works of BACON, and the mathematical classics of NEWTON and LAPLACE, are studied with a zeal and perseverance which do not fail to infuse their spirit into the minds of their aspiring successors. But in the much larger class of half-disciplined or self-taught aspirants to scientific rank, the disposition we refer to frequently exists, and to a proportionate extent retards their progress, and impairs the value of their labors.

The public teacher should, therefore, omit no proper opportunity of inculcating the true spirit of the inductive philosophy, which, in our day, has afforded so rich a harvest of discovery. I shall avail myself of the opportunity which the consideration of aerolites offers, to afford you an example of the rigorous observance of the canons of Bacon's philosophy in the investigation of nature. Every one possessed of the smallest amount of the current information of the day, imagines that he knows what meteoric stones are. He knows that they fall from the air, and that they are accompanied by fire and noise. With this amount of information he unhesitatingly sets about to conjecture their origin, and to get up a theory to explain them. As might be expected, the theory produced under such circumstances is always crude and absurd, and falls to pieces upon the slightest comparison with the phenomena.