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strument not so liable to injury as the mercurial barometer, and much more portable and easily replaced, I have taken from this paper, which will be found in the 8th number of the "Geographical Journal," the tables computed by Mr. Prinsep, to facilitate the computation of altitudes, and also the examples given by Colonel Sykes, which render their application evident without further explanation.
The results deduced from the use of these tables appear always rather less than those obtained from careful barometrical observations, and also less than those calculated from the different formulæ, which have been arranged for the determination of altitudes by this method, but which do not all agree. The results: of a number of careful observations made with the thermometer, compared with those obtained at the same time with the barometer; or which have been ascertained by levelling, or trigonometrically, will afford the means of making any necessary corrections in the tables; which, however, giving so close an approximation, deserve to be more generally known and made use of.
The accompanying sketch and explanation, taken from Col. Sykes's pamphlet, show the whole apparatus required:
A. A common tin pot, 9 inches high by 2 in diameter.
B. A sliding tube of tin, moving up and down in the pot: the head of the tube is closed, but has a slit in it, C, to admit of the thermometer passing through a collar of cork, which shuts up the slit where the thermometer is placed.
D. Thermometer, with as much of the scale
left out as may be desirable.
E. Holes for the escape of steam.
The pot is filled four or five inches with pure
water; the thermometer fitted into the aperture
in the lid of the sliding tube, by means of a collar of cork; and the tin sliding tube pushed up or down to admit of the bulb of the thermometer being about two inches from the bottom of the pot.
Before using a thermometer for this purpose, it is necessary to
ascertain if the boiling point is correctly marked for the level of the sea by a number of careful observations, and the difference, if any, must be noted as an index error. It is always desirable to have two or more thermometers which have been thus tested; and in all observations the temperature of the air at the time should be noted.
TO FIND THE BAROMETRIC PRESSURE AND ELEVATION CORRESPONDING TO ANY OBSERVED TEMPERATURE OF BOILING WATER BETWEEN 214° AND 180°.
TABLE OF MULTIPLIERS TO CORRECT THE APPROXIMATE HEIGHT FOR THE TEMPERATURE
When the water (with the thermometer immersed) has been boiled at the foot and at the summit of a mountain, nothing more is necessary than to deduct the number in the column of feet opposite the boiling point below, from that opposite the boiling point above this gives an approximate height, to be multiplied by the number opposite the mean temperature of the air in Table II., for the correct altitude.
Boiling point at summit of Hill Fort of
Púrundhur, near Púna
Boiling point at Hay Cottage, Púna
Temperature of the air above.
When the boiling point at the upper station alone is observed, and for the lower the level of the sea, or the register of a distinct barometer is taken; then the barometric reading had better be converted into feet, by the usual method of subtracting its logarithm from 1.47712 (log. of 30 inches) and multiplying by 6, as the differences in the column of "barometer" vary more rapidly than those in the "feet" column.
Assuming 30.00 inches as the average height of the barometer at the level of the sea (which is however too much), the altitude of the upper station is at once obtained by inspection in Table I., correcting for temperature of the stratum of air traversed, by Table II.
In moderate elevations, the difference of one degree in the temperature at which water boils, indicates a change of level of about 500 feet, nearly equivalent to what would be shown by a difference of 0·6 of an inch in a mercurial barometer.
SHADING AND ENGRAVING TOPOGRAPHICAL PLANS.
AFTER all the mechanical portions of the survey, (including the horizontal contours if they have been traced instrumentally,) have been plotted to the required scale; the features of the ground, and any other detail that may have been sketched in the field, are transferred to the original plot for the commencement of the finished plan, supposing one to be required either to be preserved as a drawing, or for the purpose of engraving. This is generally finished with a brush, either in Indian ink or sepia; but a grea want of one general system of topographical plan-drawing is here felt, particularly as regards the method of expressing the features of the ground in a manner at once easy of execution and generally intelligible.
The different disposition of the light affords the means of varying the system of shading hills. Where it is supposed to descend in parallel vertical rays upon the ground, each slope evidently receives less light, or, relatively speaking, more shade, in proportion to its deviation from a horizontal plane, on which the maximum of light falls. Mr. Burr, in his "Practical Surveying," devotes a chapter to the scale of shade to be applied to plans finished on this supposition, which however he candidly acknowledges to be an impracticable theory; but it leads him to the very just conclusion, that hills are generally shaded much too dark to give anything like a natural representation of their various slopes, which defect has also the additional fault of confusing the appearance of the drawing, and impairing the accuracy of the outline. The slopes drawn upon this system have evidently no light or dark sides, which causes a monotonous effect; and yet, on the same plan, both trees and houses are constantly represented with shadows.