805.—Oil Lanterns.—In the great trigonometrical survey of India large reverberatory lamps were used, which were furnished with Argand burners with circular wicks about 2 inches in diameter. The back arc of rays was reflected by a parabolic reflector 12 inches in diameter and 4·9 inches in depth. The lamp was enclosed in a strong box with a plate-glass face 12 inches in diameter, with apertures to admit sufficient air and chimney to carry off fumes. The box was constructed to form a packing case for conveyance of the apparatus.[58]
806.—The oil lantern which will be found most convenient for the civil engineer will be one of the same form of construction as the bull's-eye lantern, but much larger—6 inches square is a good size. This may be made to go on the same tripod as the heliograph, and will take its place for signalling by night, or telegraphing by the Morse signals by the hand or bat shown [Fig. 387], D. A 6-inch bull's-eye lamp with treble wick may be seen well in clear weather 5 miles to 10 miles off. A railway signalman's hand lamp forms a very good signal, or even an ordinary 4-inch bull's-eye is very useful in working over new countries.
807.—Magnesium.—The intense light given by burning ribbon magnesium, and the extreme lightness in weight of this material, render it of especial value for night signalling. Magnesium ribbon is now sold at a very low price (about two shillings per oz.), and 1 oz. will give a continuous intense light, visible at 30 miles, for over an hour, whereas for a night signal arranged to be given at a stated time, fifteen minutes is amply sufficient for a single observation. Great difficulty is often found in lighting magnesium ribbon when this is slightly oxidized from exposure to air. The best method is to employ the flame of a portable spirit lamp, made for the purpose. Under any condition the burning ribbon should be shaded from wind. A common plan is to hang a straight slip of ribbon from the centre of a tripod which can be readily shaded by a pocket handkerchief. Where expense is not the object to be considered, lamps may be had for burning the wire. Tin cases are made for soldering up and storing the ribbon in for use abroad.
CHAPTER XVIII.
MEASUREMENT OF ALTITUDES BY DIFFERENCES OF ATMOSPHERIC PRESSURE—HISTORICAL NOTE—MERCURIAL BAROMETER—CONSTRUCTION—OPERATION—ANEROID BAROMETER—CONSTRUCTION—VARIOUS IMPROVEMENTS—HYPSOMETER.
808.—Historical Note.—The observation that the atmosphere decreases in density with increase of height is due to Alhazen the Saracen, about a.d. 1000. By this he explains that a ray of light entering the atmosphere obliquely follows a curvilinear path, bending towards the denser strata, that is concave towards the earth. He showed that a body will receive difference of pressure in a rare and a dense atmosphere, and calculated that the height of the atmosphere to its final attenuation would be from his data nearly 58½ miles. The practical instruments that have been devised for measuring altitudes, by the differences of pressure due to the weight of superincumbent atmosphere are the barometer, the aneroid, and the hypsometer. The barometer was invented by Torricelli about the year 1640. Its principle was demonstrated and first applied to altitude measurement by Pascal in 1647. The aneroid barometer was suggested by Conti in 1798, and said to be devised as a practical instrument by Vidie in 1808. The hypsometer or boiling-point thermometer, which depends for its boiling temperature upon the pressure of the atmosphere above the liquid which surrounds it, was suggested by Fahrenheit in 1724, experimented with by de Luc in 1772, and brought to its present practical form by Regnault about 1840. At the present time the aneroid is almost exclusively used by the civil engineer, as this instrument when made with great care is sufficiently reliable, more portable, and not so delicate in use as the others. So that it is only when very great precision is desired, or when the one instrument is used as a check upon the other, that the mercurial barometer, or the hypsometer, or both are now employed. At the same time it must be understood that the aneroid barometer scale is in a certain degree arbitrary, as the divisions at best are only made up from a certain number of points taken from observations of the mercurial barometer placed simultaneously with the aneroid under an air pump, and therefore its errors comprise those of the particular mercurial barometer with which it is compared, and those due to the difficulties of the comparison, and of making subdivision afterwards in the same relative proportion, by copying to the scale of the aneroid.
809.—The Mercurial Barometer.—The principle of the barometer is generally understood. If a glass tube, closed at one end, 33 inches long, say of ¼ inch or over in bore, be filled brimful of mercury and the point of the forefinger be firmly pressed on the surface of the mercury, the tube may be inverted without the admission of air. If the covered end of the tube be now plunged into a basin of mercury and the finger slowly withdrawn from under the tube beneath the surface of the mercury, the latter will sink in the tube to about 30 inches above the surface of that in the basin—that is, if the experiment be performed at about the sea level. The empty space in what now becomes the top of the tube is termed a Torricellian vacuum.
810.—In removing the pressure of the atmosphere from its surface in the tube, which in the above experiment produces the barometer, the pressure of the atmosphere then falls only upon the exposed surface of the mercury in the basin, or what is technically termed the cistern. This pressure is equal per area, according to hydrostatic laws, to the upper surface area of any equal column of mercury that the barometer may contain. Therefore the weight of the column of mercury in the tube, if cylindrical, above the surface of that in the cistern, is the same as that of a column of air of equal size reaching upwards to the full height of the atmosphere. In fact the one exactly balances the other, and it is by the difference of the weight or quantity of air above the barometer per area of bearing surface that it is possible to ascertain the altitude of its position by means of the height of mercury in the tube, after proper allowance is made for sudden changes of conditions of the atmosphere itself from time to time, capillary attraction of the tube, temperature, etc.