BAROMETRICAL MEASUREMENT BARON 331 ing two days, the liability to error resulting from too great distance was avoided ; so also was that from a faulty correction for tempera- ture. This correction, as applied by the tables, amounts sometimes to 300 ft. But it supposes the actual temperature of the stratum of air between the two points to be represented by the mean of the temperature at the two places, and a moderate variation from this may well involve an error of -^ or ^ of the whole cor- rection. Such a variation is not at all improb- able where the difference of elevation is very great, as in the higher regions the decrease of temperature takes place more and more rapidly. The next station was the summit of the first hill, the height of which was ascertained by comparative observations made upon it and at the same time at the second station. The dif- ferent peaks were then compared one with an- other by observations made upon them in pairs. So exactly were these measurements conducted by Prof. Guyot, that, as he states, his single observations differed only two or three metres from the means, and the mean of one day scarcely differed one metre (39 inches) from the mean of another. But for these precau- tions an error might have resulted in the de- termination of the first summit of 50 ft. or more, such as Prof. Guyot found he was liable to in the course of his observations at the White mountains when the two stations were from 10 to 20 m. apart. As the distance between stations increases, the number of observations should also be multiplied, in order to obtain a correct mean. The barometers are to be care- fully suspended, so that the column shall be perfectly vertical, and they should be placed in a situation not subject to sudden change of temperature. The reading of the height of the mercurial column is to be taken at the same time as that of the thermometer attached to the barometer, and also of the detached ther- mometer. If the instrument has been suspend- ed for some moments, the two temperatures may not differ. When these observations are compared with those made at the same time at the other station, the calculations for the dif- ference of elevation are usually made by the aid of the tables prepared by M. Oltmanns. This is a much more simple process than calcu- lating the difference by the theorem of Laplace, which gives the same result. If the instru- ments are graduated in inches, these must be turned into metres, and the temperatures must also be expressed in degrees of the centigrade thermometer. With the tables for these con- versions and calculations are given very simple directions for their use, and applying the neces- sary corrections. Some singular barometric anomalies are reported by Lieut. Herndon to have been observed by him in the vicinity of the Andes. At the eastern base he found the pressure, as measured by the boiling point of water, to be nearly as great as at the level of the sea. Having descended nearly 1,000 m. on the Amazon, the boiling point indicated an as- cent of nearly 1,500 ft. Maury explains this by referring it to the effect of the trade winds, which strike upon the flanks of the mountains and are banked up against them, as a current of water interrupted by impediments in the chan- nel is piled against these. By the banking of the current of air an increased pressure is sup- posed to be exerted upon the surface at their base. In the earlier measurements made with the barometer the air was considered as a uni- form fluid, no regard being paid to the gradual diminution of density in ascending into the higher regions ; but when this gradation was taken into the calculations, it became neces- sary to determine the relation between the j density of the air and its elastic force. Mari- otte, who published his " Discourse on the Na- I ture of Air" in 1076, and who was the first I to demonstrate the law which bears his name, j that the volume of a gas is in the inverse pro- portion to the pressure upon it, opened the
- culture of a new field from which rich harvests
i were subsequently reaped. From the sugges- 1 tions afforded by this simple law he proposed to compute heights from barometrical observa- tions by the rule usually employed in con- structing tables of logarithms, seeming to have obtained some idea of the remarkable fact that the density of the atmosphere decreases in a geometrical progression corresponding to the elevations taken after an arithmetical one. But for some reason he seemed not to be aware of the importance of the great principle, and abandoned the method for another in which he repeated the bisection of a column of air between two stations into successive horizontal strata, calculating the densities according to a harmonic division. BARON (Gallic ber, Gothic, vair, medieval Latin baro, early Spanish varon, a man), in the middle ages, the possessor of an estate, who might have feudal tenants under him. In France the nobles in general were at first called barons, but subsequently the immediate vassals of the king received the appellation of hauls barons, or high barons. In Germany the early barons were the highest nobility, who after- ward assumed the titles of counts and princes. In more modern times, in both France and Ger- many, a baron (in the latter country now gen- erally called Freiherr), is a nobleman next in rank to a count. In England the original bar- ons of the realm were those who held lands by tenure of suit and service to the king. They were bound to attend the king in war, to supply money on particular occasions, to furnish a mil- itary contingent proportioned to the extent of their fiefs, and to attend the king's courts. Va- rious circumstances having increased the num- bers of the barons holding direct from the sove- reign, a practice became established about the time of Edward I. of summoning individuals by writ to the great councils. The barony by tenure and by writ being heritable, the inher- itance of the titles became complicated by the devolution of the estates to female descendants,
