land, and strike their way across the great | inch, together with the The ordinary barometer in use by Signal Office observers is that of Mr. James Green (the well-known scientific instrument maker of New York)-an instrument adopted by the Smithsonian Institution, and also by the American navy, as the most perfect to be obtained. This barometer has its cistern furnished with a small glass index, which shows when the mercury is at the right height in the cistern. This is adjustable by a screw which works through the bottom of the instrument against the flexible bottom of the cistern. The instrument is ready for use when the mercury touches the little V-shaped index in the cistern. So simple and complete is this barometer that any one can use it, and it ought to be in the hands of all business gentlemen, and all who are interested in watching the mutations of weather. In reading the barometer a vernier is used. The vernier (Figs. 1 and 2) consists of a piece similar to the scale of the barometer, and along which it slides. It will be seen from Fig. 1 that ten divisions of the vernier are exactly equal to eleven divisions of the scale; that is, to eleven-tenths of an inch. Each division of the vernier is, therefore, equal to a tenth of an stand as in Fig. 2, we read thus: (1) The zero of the vernier being between 29 and 30, the reading exceeds 29 inches, but less than 30 inches. Hence the first figure is 29 inches. (2) Counting the tenths of an inch from 29 upward, we find the vernier indicates more than 7-10ths and less than 8-10ths, giving the second figure, 7-10ths. Casting the eye down the scale to see the point at which a division of the scale and one of the vernier meet in one and the same straight line, we find it at the figure 6-the last figure. And we read the barometer 29.76. By this simple mechanical contrivance the barometer is read to so fine a degree that the variation of 1-100th of an inch in the mercurial column is detected! BAROMETRIC OSCILLATIONS. Latitude and longitude on the earth's surface mark very conspicuous differences in the mean barometric pressure, as will be seen by a study of the Isobarometric Chart for the United States. The barometer has a slight fluctuation also under several influences. It rises when the moon is on the meridian in some places. It has a diurnal oscillation, amounting on the equator to more than one-tenth of an inch, but in the latitude of New York to only 0.05 inch, the greatest height being about 10 A.M., and the least about 4 P.M. The nocturnal variations are much less. In the latitude of Phila delphia and New York the northeast wind causes another variation of one-fourth of an inch, due to the meeting of two atmospheric waves giving a still higher wave, and hence a higher barometer. There is also the variation due to the height of the observer's station above the sea. This is, of course, of the first importance. The other fluctuations are comparatively unimportant, and do not blind an observer to those ominous fluctuations which precede the storm, the tornado, and the hurriThe oscillations which indicate a storm are very marked. The tornado which recently ravaged St. Louis was preceded by a gradual fall of the mercury in the barometer, for thirty hours previous, of an entire inch. At Boston, within thirty-seven years, the barometer has ranged from 31.125 inches to 28.47 inches, the difference being 2.655 inches. At London it has ranged through more than 3.5 inches; but in the tropics not so much. cane. During the passage of a cyclone the mercury oscillates rapidly. The most noticeable fall occurs from four to six hours before the passage of the storm centre. This fall is often over an inch, and sometimes two inches. Great changes are usually shown by falls of barometer exceeding half an inch, and by differences of temperature exceeding fifteen degrees. If the fall equals one-tenth of an inch an hour we may look out for a heavy storm. The more sudden the change the greater the danger. But it is too often forgotten that the fall of the mercury is a forewarning of what will occur in a day or two, rather than in a few hours. A variation of an inch is certain to be followed by a tornado or violent cyclone. In the tropics "the glass" has been known to show a fall of more than an inch and a half in one hour! The following guides in predicting weather changes are selected from the "Barometer Manual" of the London Board of Trade, and are suggestive: I. If the mercury standing at thirty inches rise gradually while the thermometer falls, and dampness becomes less, N.W., N., or N.E. wind; less wind or less snow and rain may be expected. II. If a fall take place with a rising thermometer and increasing dampness, wind and rain may be expected from S. E., S., or S. W.; a fall in winter with a low thermometer foretells snow. III. An impending N. wind before which the barometer often rises may be accompanied with rain, hail, or snow, and so forms an apparent exception to the above rules, for the barometer always rises with a north wind. IV. The barometer being at 29 inches, a rise foretells less wind or a change of it northward, or less wet. But if at 29 inches a fast first rise precedes strong winds or squalls from N.W., N., or N.E., after which a gradual rise with falling thermometer, a S. or S.W. wind will follow, especially if the rise of the barometer has been sudden. V. A rapid barometric rise indicates unsettled, and VI. The greatest barometric depressions indicate gales from S. E., S., or S. W.; the greatest elevations foretell wind from N.W., N., or N. E., or calm weather. VII. A sudden fall of the barometer, with a westerly wind, is sometimes followed with a violent storm from the N. W., N., or N.E. VIII. If the wind veer to the S. during a gale from the E. to S. E., the barometer will continue to fall until the wind is near a marked change, when a lull may occur. The gale may afterward be renewed, perhaps suddenly and violently; and if the wind then veer to the N. W., N., or N.E., the barometer will rise and the thermometer fall. IX. The maximum height of the barometer occurs during a northeast wind, and the minimum during one from the southwest; hence these points may be considered the poles of the wind. The range between these two heights depends on the direction of the wind, which causes, on an average, a change of half an inch; on the moisture of the air, which produces in extreme cases a change of half an inch; and on the strength of the wind, which may influence the barometer to the extent of two inches. These causes, separately or conjointly with the temperature, produce either steady or rapid barometric variations, according to their force. SELF-REGISTERING INSTRUMENTS. But invaluable as is the ordinary barometer which has been described, the most valuable instruments are those which are automatic, or self-registering. Prominent among these are the celebrated self-recording barometer and the meteorograph invented by Professor G. W. Hough, Superintendent of the Dudley Observatory at Albany. Lord Rosse's telescope has not done more for astronomy than will the self-registering barometer do for meteorology. Through the great kindness of Professor Hough, in sending me wood-cuts of his beautiful inventions, I am enabled to present these simple yet complete and consummate contrivances. The diagram, Fig. 1, page 410, will illustrate the method of registering the height of the barometer and thermometer on a single sheet by the use of one set of mechanism : Let D be a drum 6 inches in diameter and 7 inches in height, covered with a sheet of ruled paper. This drum is presumed to revolve at any convenient rate, say 1 inch per day. Let L be an iron or brass bar 24 inches in length, mounted on an axis passing through the point c. Let P be a steel pen attached to the end of the lever projecting over the centre of the drum. Let P' and P be platinum wires attached to the lever at 3 inches on either side of the axis c. The wire P' is over the shorter leg of a siphon barometer, and the wire P" passes into the end of an open mercury thermometer. Now if the lever L be elevated at the end over the drum, the wire P' will touch the top of a float resting in the shorter leg of the siphon barometer. If then a battery, B, and electro-magnet, E, be arranged as in the diagram, when contact is made with the float a current of electricity will pass through the circuit, and the electro-magnet E is operated. If then, when the circuit is completed, a blow be struck on the pen P, by means of the electro-magnet, or a hammer unlocked by it, the dot on the drum sheet will indicate the height of the barometer at that time. It is obvious that as often as the lever is elevated a record will be made. For the barometer an hourly record will be found to be sufficient. If the lever L is rigid and firmly mounted, the mere Fig. 1.-REGISTRATION OF THE HEIGHT OF BAROMETER AND THERMOMETER. measurement of height by means of electrical contact can be carried to almost any degree of precision. It was found from numerous experiments made some years since that the magnetic circuit is not completed for a distance of one-tenthousandth of an inch. Therefore, whatever source of error there may be in the results recorded by this method is due to the barometer itself. In practice, from records extending over nearly one year, it is found that the results are inside the errors of reading from the drum sheet. A long experience has led to the conclusion that this degree of precision is sufficient for the investigation of barometric changes, and is but little outside the limit of error from reading a standard barometer. An examination of the diagram will also show at a glance how the height of the thermometer is recorded. It should, however, previously be stated that the thermometer is a little larger than those in ordinary use, and has a platinum wire, a, cemented in the bulb, communicating with the mercury in the inside. DESCRIPTION OF A NEW METEOROGRAPH. (WEATHER RECORDER.) The following is a general description of a machine constructed for the Signal Service at the request of the chief signal officer. It registers hourly the barometer and wet and dry bulb thermometers, and thus shows the atmospheric pressure, the temperature of the atmosphere, and its hygrometric conditioni. e., its condition of moisture or dryness. The engraving, Fig. 2, page 411, is a perspective view of this instrument. The recording lever, A, is a bar of iron about two feet in length, nearly balanced on the axis, supported by the clock-frame, C. The clock is constructed with rather stronger gearing than an ordinary movement, its office being to elevate and depress the lever A hourly, regulate the drum, D, and raise the two striking hammers, H and H'. It is provided with a half-second pendulum, and requires winding once in two days, the weight dropping in that time about three feet. The shorter leg of the siphon barometer is shown at B, and the wet and dry bulb thermometers at T' and T. Directly over the leg of the siphon, as also over the two thermometers, the lever A supports a carriage, which is depressed or elevated whenever the lever A is in motion. The registering point, G, is connected with the lever, as shown in the diagram; and the curvilinear motion of the end of the lever is converted into rectilinear by allowing G to slide against a vertical steel rod. To illustrate the action of the machine, we will suppose the lever A has reached its low BAROMETER Fig. 2.-THE METEOROGRAPH. est point, the registering pen G being at the bottom of the drum. Now, in order that we may be able to register the barometer on any part of the drum sheet, it is necessary that the striking hammer should be elevated and locked before the upward motion of the lever commences. As the hammers are raised by means of an arm carried by the hour shaft of the clock, at the point where the hammers begin to rise the snail for elevating the lever A is cut away, so that it remains at rest during a period of fifteen minutes, the time required for elevating the hammers H and H'. As soon as this is accomplished the lever begins to rise slowly, by means of the double snail on the hour shaft, the time required for traversing the drum being about fifteen minutes. When the position of the lever is such that the carriage in the rear of the clock touches the float in the shorter leg of the siphon, an electric current is established through the magnet, F, which un- ures of the invention of Professor Hough is that locks the hammer H, causing the pen G to it prints its own records. And this is done by make a record on the drum sheet. After the a single screw, which rises or falls with the merlever has reached the top of the drum it re-cury in the barometer. This screw carries a mains at rest fifteen minutes, while the ham- pencil, which traces upon a revolving cylinder mers are being raised, when it is gradually de- or roll of paper a line showing the minutest pressed. So soon as the platinum wires-at- movements of the column of mercury for every tached to the carriage over the thermometers-minute in twenty-four hours. This same screw touch the surface of the mercury in the ther- also gives motion to a series of wheels which carry mometer tubes, electric currents are established types, by which, at the end of every hour, the height through the magnets F and J, simultaneously of the column of mercury is printed on a slip of or successively unlocking the hammers, and, as paper to the accuracy of THE THOUSANDTH PART the case may be, making records as before. OF AN INCH! A complete double motion of the lever requires one hour. During this time the barometer and wet and dry bulb thermometers have each been recorded once. The records of the barometer and thermometers differ in time about half an hour. The wet and dry bulb thermometers are recorded within about one minute of each other, depending on the difference between them. One of the most beautiful and simple contrivances used is a Wild's self-registering barometer, of which we give a cut one-quarter the actual size. It scarcely needs explanation except to say that the tube, A, is suspended in a cistern of mercury, represented on the left of Fig. 1. As the atmospheric pressure changes, the level of the mercury changes in the cistern, and the tube A rises or falls as the atmospheric pressure One of the most marked and wonderful feat- increases or diminishes. The weight of this |