a number of careful observations obtained by both methods are compared together, or with a trigonometrical or levelling process, they will afford the means of making any necessary Lieut. Robinson and Mr. Prinsep's Tables give close approximations to each other in their results, but they are so much below the corresponding barometric observations, which I consider the true heights, as the results by the Madras Table are above the true heights. Some of them curiously coincide within a foot or two of the heights determined by corresponding barometrical observations, but this coincidence must be the result of mere accident. Taking the mean of all the thermometric observations at a station calculated by the three Tables, and the mean of all the corresponding barometric observations at the same place, the utmost difference is 107 feet in less than 600; and the least difference is 8 feet in about 3000; but as the thermometric heights, in which the difference of 107 feet occurs, were single observations, made by a gentleman who had newly begun to use his Thermometers, they may be looked upon as probably less accurate than subsequent trials would have made them. This is scarcely an unjust inference, as it will be seen that the next greatest difference made by the same gentleman was only 24 feet in 4490. It must be admitted, however, that this amount of error is just as likely to occur in heights of 100 feet as in those of 10,000. My Thermometers were not graduated to less than half-degrees, and long practice enabled me to determine the height of the mercury in the steam to one-twentieth of a degree; but I would recommend Thermometers being used in which the degrees are graduated to fifths or tenths of a degree. On the whole, I think the results of six years' experience justify me in saying that common Thermometers may be satisfactorily used to supply the place of Barometers in measuring heights where great accuracy is not required; and it will be recollected that what is usually looked upon as a difficult and troublesome operation with Barometers, will be attainable by any person who carries with him a couple of Thermometers, the requisite tin pot, and the Tables, and who is master of the simplest rules of arithmetic. Of the three Tables in my possession, I have chosen Mr. Prinsep's from their perspicuity and the facilities they offer for the conversion of boiling temperatures into heights with very little trouble; but a glance over the figures in my Tables of Altitudes will show that the Tables are susceptible of considerable improvement, for with two exceptions, all the heights deduced from Mr. Prinsep and Lieut. Robinson's are much below those determined by simultaneous observations with good Barometers; and I join with Mr. Prinsep in expressing a hope that every traveller boiling his Thermometers will, at the same time, if he possess a Barometer, make a record of its indication, and thus render essential service to physics, by fixing so many points on the scale of the elastic tension of steam at different temperatures. corrections in the Tables. The approximation, however, is sufficiently close in the examples given in Colonel Sykes' book, as to induce great confidence in the method, and espe TABLE. I. To find the Barometric Pressure and Elevation corresponding to any observed Temperature of Boiling Water between 214° and 180°. cially for determining the comparative altitudes of places in a mountainous country. The Tables being given in degrees of Fahrenheit, it will be necessary in case Centigrade Thermometers are used, to convert these indications into the corresponding ones of Fahren 9c heit, for which the formula is F= +32 whenever the 5 TABLE II. Table of Multipliers to correct the Approximate Height for the Temperature Enter with the mean temperature of the stratum of air traversed, and multiply the approximate height by the number opposite, for the true altitude. degrees are above the freezing point of water, and vice versa, for converting Fahrenheit into Centigrade measure—the (F-32) x 5 9 formula will be C= When the Thermometer 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 the same of 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 204-2 Temperature of the air above.... feet. = 4027 208.7 = 1690 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 0006, as the differences in the column of Barometer' vary more rapidly than those in the 'feet' column. Example.-Boiling point at upper station.......... Barometer at Calcutta (at 32°) 29 in. 75° 185° 14548 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 of Table I., correcting for temperature of the stratum of air traversed by Table II. CHAPTER XXIII. ON LEVELLING. LEVELLING is the art of tracing a line at the surface of the earth, which shall cut the directions of gravity everywhere at right angles. If the earth were an extended plane, all lines representing the direction of gravity at every point on its surface would be parallel to each other; but in consequence of its figure being that of a sphere or globe, they everywhere converge to a point within the sphere which is equi-distant from all parts of its surface; or, in other words, the direction of gravity invariably tends towards the centre of the earth, and may be considered, as represented by a plumbline when hanging freely, and suspended beyond the sphere of attraction of the surrounding objects. B I A D E In the above diagram let the straight line AB represent the surface of the earth, upon the supposition of its being an |