water it meets with in its passage to the ground, but the amount thus collected would not be sufficient to account for the difference observed. Besides this, the condition does not always exist; the air near the earth is frequently undersaturated during rain, and in this case a portion of the drop would be evaporated, and its size on reaching the earth less than it was above. If the drop is increased by the deposition of new vapor in its descent, then the rain at the bottom ought to be warmer than at the top, on account of the latent heat evolved in the condensation; on the other hand, if the drop be diminished by evaporization during its fall, then the temperature of the rain caught at the greater elevation ought to be in excess. That evaporization does sometimes take place during the fall of rain, would appear from the fact that clouds are seen to exhibit the appearance of giving out rain though none falls to the earth, the whole being entirely evaporated. That the air should ever be undersaturated during rain is at first sight a very surprising fact; it may, however, be accounted for on the principle of capillarity. The attraction of the surface of a spherical portion of water for itself is in proportion to the curvature or the smallness of the quantity, and hence the tendency to evaporate in a rain-drop ought to be much less than in an equal portion of a flat surface of water.

If the diminution of quantity of rain at the upper station depends principally on eddies of wind, then the effect will be diminished by an increase in the size of the drops, which will give them a greater power of resistance; and the size of the drop will probably be influenced by the intensity of the electricity of the air, as well as by its dryness. The former, as well as the latter, will tend to increase the evaporation from the surface of the drop.

It is a well-established fact, which at first sight would appear to be at variance with the results of observations on towers, that a greater amount of rain falls in some cases on high mountains than on the adjacent plains. For example, the amount of water which annually falls at the convent of St. Bernard is very nearly double that which falls at Geneva. This effect, however, is due to the south wind, loaded with moisture, ascending the slope of the mountain into a colder region, which causes a precipitation of its vapor. From what is here said, it will be evident that the subject of rain is one which involves many considerations, and which still presents a wide field for investigation.

A series of observations have been commenced at this Institution on the quantities of rain at different elevations, as well as on gages of different sizes and forms, the result of which will be given in one of the subsequent reports.

METEOROLOGY.

DIRECTIONS

FOR

METEOROLOGICAL OBSERVATIONS,

ADOPTED BY THE SMITHSONIAN INSTITUTION, FOR THE FIRST CLASS OF OBSERVERS.

The following directions were originally drawn up for the use of the observers in correspondence with the Smithsonian Institution, by Professor GUYOT, of the College of New Jersey, Princeton, and are now reprinted, with a series of additions, for more general distribution. The additions are indicated by brackets, [ ].

SECRETARY S. I.

PLACING AND MANAGEMENT OF THE INSTRUMENTS.

THERMOMETER.

Placing. Place the thermometer in the open air, and in an open space, out of the vicinity of high buildings, or of any obstacle that impedes the free circulation of the air. It should be so situated as to face the north, to be always in the shade, and be at least from nine to twelve inches from the walls of the building, and from every other neighboring object. The height from the ground may be from ten to fifteen feet, and, as far as possible, it should be the same at all the stations. The instrument should be protected against its own radiation to the sky, and against the light reflected by neighboring objects, such as buildings, the ground itself, and sheltered from the rain, snow, and hail. The following arrangement will fulfil these requirements, (figure 1.)

Select a window situated in the first story, fronting the north, in a room not heated or inhabited; remove the lattice blinds, if there be any, and along the exterior jambs of the window place perpendicularly two pieces of board, (a b-a' b'), projecting to a distance of from twenty to twenty-four inches from the panes. At half this distance, ten or twelve inches from the panes, and at the height of the eye of the observer, when in the chamber, pass from one piece of board to the other two small wooden transverse bars, (c d, dd',) each an inch. broad, for the purpose of supporting the instruments. Upon the outer edge of the boards fasten in the usual way (H H) the latticed blinds which were removed from the jambs, or two others provided for the purpose. That blind, behind which the instruments are to be placed, is to serve as a screen, and must be fastened, almost entirely closed, so as make a little more opening; the other will remain entirely open to allow a free access of air and light, and is not to be closed except in great storms. The whole must be covered with a small inclined roof

of board, (B E,) placed at least fifteen or twenty inches above the instrument. The lower part, (J J,) or the basis, may remain open.

[The foregoing is a convenient arrangement by which the observations can be taken without exposing the observer to the weather. To insure greater accuracy the windows during the intervals of observations may be closed with a wooden shutter. The outside of the lattice work should be painted white, to reflect off the light and heat which may fall upon it.]

Fig. 2.

The thermometer must be placed exactly perpendicular, the middle of the scale being at the height of the eye against the two small wooden bars, so that the top of the scale being fixed by a screw to the upper bar, the bulb may pass at least two or three inches beyond the lower bar. The instrument is attached to the last by a little metallic clasp. (Fig. 2.) It will thus be placed ten or twelve inches from the panes, from the screen, and the other parts of the window.

[In a later arrangement, a single transverse bar is used. This being placed at the necessary height, the thermometers are attached to it by small metal brackets, which support them at a distance from the bar of about two inches. The metal brackets are permanently screwed to the bar, and the thermometers are fastened to them by small finger-screws, by which they can be detached at pleasure. The order of placing them is shown in the cut.]

Reading. To read the thermometer, the eye must be placed exactly at the same height as the column of mercury. Unless this precaution is taken, there is a liability to errors, the greater in proportion to the thickness of the glass of the stem and the shortness of the de

grees. The reading should be made at all times, and especially in the winter, through the panes, and without opening the window; otherwise the temperature of the chamber will inevitably influence the thermometer in the open air. The degrees must be read, and the fractions carefully estimated in tenths of degrees. After having rapidly taken the observation, another should be made to verify it. If there are several other instruments to observe, and the thermometer is to be read first, the first reading may be made some minutes before the hour; the second, after the reading of the psychrometer; and if there is a difference, the mean number is to be entered in the journal. When, notwithstanding the shelter, the bulb of the thermometer is moistened by rain or fog, or covered with ice or snow, it is necessary to wipe it rapidly, and not to record the degree until the instrument has been allowed to acquire the true temperature of the air.

Verification.-Verify the zero point, at the beginning and end of winter. For this purpose, fill a vessel with snow, immerse the bulb of the thermometer in the middle of it, so as to be surrounded on every side by a layer of several inches of snow, slightly pressed around the instrument. The stem must be placed exactly perpendicular, and covered with snow as far up as the freezing-point on the scale. Let it stand so for half an hour or more, and then read it, taking great care to place the eye at the same height as the summit of the mercurial column. If the top of the column does not coincide with the freezing-point of the scale, the exact amount of the difference must be ascertained, and the correction immediately applied. At the same time enter in the journal, under its appropriate head, the day on which the experiment is made, its quantity, and the moment at which the application of it was commenced. [It is necessary to add that since the zero point of the thermometer is not that of the temperature of snow as it is frequently found when exposed to the atmosphere, but that of melting snow, the experiment must be made in a place above the temperature of freezing. Instead of snow, pounded ice may be employed.]

[Green's thermometers have an arrangement by which the tube can be slipped down the small quantity necessary to correct for this change. The end of the tube is fitted into a small plate of German silver, and this fastened by a screw to the scale. If, on testing the thermometer, the mercury be found to stand above 320, free the screw one or two turns without taking it out, and push down the plate the necessary amount to bring the mercury to coincide. The thermometer must be handled with great care in making this adjustment, and it may be well, for additional security against accident, to loosen all the screws which fasten the bands around the tube-it will then slide in them more freely. After completing the adjustment, they may again be set moderately tight. The object of this adjustment being only to avoid the trouble of making a correction, it is not advisable to attempt it, if the observer thinks that he risks, in so doing, the safety of his instrument. As the tubes of these standard ther

mometers are kept for a considerable time before fixing the zero point, in most cases the moving will not be required. After the first year the zero point changes little, and practically, when exposed only to atmospheric influences, may be considered permanent.]

SELF-REGISTERING THERMOMETERS.

Placing. These two thermometers, indicating the maxima and minima, are to be placed beside the common thermometer, in a horizontal position, with the bulbs opposite and free, on two small perpendicular supports uniting the two bars, as shown in Fig. 1.

Reading. For the reading, place the eye in such a position that the visual ray may be perpendicular to the extremity of the index; enter the indications with the fractions of degrees, if there are any, and, after having verified them again, bring back, by means of the magnet, the indexes of the two thermometers to the summit of their respective columns.

Verification.-Compare the indications of the two thermometers frequently, and especially the spirit thermometer, with those of the common thermometer; verify the zeros at least twice a year, and, if there is a difference, adjust the zero anew, if the instrument permits, to eliminate the correction, as has been stated above for the simple thermometer, or take this correction into account in the register.

[The maximum thermometer is subject to derangement by the mercury getting to the side of the steel index and wedging it fast. When such is the case, put the bulb in ice, if it is necessary to bring the mercurial column so low, or cool it sufficiently to get all the mercury down that will pass the index; then move the magnet along the tube with a slight knocking or jarring motion, and try to get the index into the chamber at the top of the stem. If you get the index free of the wedge, but with mercury above it, heat the bulb until all the disjointed mercury and index are driven into the chamber, then keep the index up by the magnet, and the mercury will go back as the bulb cools. The great point of attention is to get and keep the index free of the wedge. The mercury being above, is of little consequence, as it can readily be heated up into the chamber; in doing this, most watchfulness is required in not suffering the index to wedge by the driving mercury. If the index is so wedged that it cannot be moved by these methods, then take the thermometer steadily in the hand, and swing it quickly, as if you wished to throw the mercury into the chamber at the fop; the index with more or less mercury will be found in the chamber: if not, repeat the swinging until it is there. Then heat up the bulb until the mercury joins that in the chamber, keep the index up by the magnet, and let the mercury by cooling go back in unbroken line.

In using the magnet to move the index up into contact with the mercury, care must be taken not to urge it too strongly, or it may enter the mercury.

In using the magnet with the spirit-thermometer, the same care is necessary as with the mercurial, as the index may be forced out of the spirit, entangling the vapor and the alcohol. When this is the case the thermometer must be taken down and held vertically-a few taps or jars will bring the spirit together. The spirit-thermometer requires attention, also, in this particular. The vapor above the spirit. is apt, in time, to condense at the end of the tube, commonly at the very end. When the spirit-thermometer stands lower than the mercurial one, this may be suspected and looked for. When so found,