428 BAROMETRICAL MEASUREMENT OF HEIGHTS. 



Table XIX. gives, in metrical measure, the values of a millimetre in the barom- 

 eter at different elevations and Centigrade temperatures. The values are derived 

 from Laplace's constants, as in Tables XVI. and XVII. 



This table may be used, as the preceding ones, for reducing barometrical obser- 

 vations to the level of the sea, and also to any other level by a similar process. 



Example. 

 Suppose the barometer to read 700 millimetres at the altitude of 750 metres, the 

 temperature of air being = 16° Centigrade ; what would be the reading at a station 

 lower by 350 metres, assuming the temperature of the air downwards to increase at 

 the rate of 1° Centigrade for 185 metres .'' 



The temperature of air at lower station will be 16° -)- 1°.9 = 17°. 9 

 The a[)proximate height of barometer about 73 centimetres. 



Then, in Table XIX. we find for 16° and 70 centimetres, 12.15 - 

 " " for 17°.9 and 73 centimetres, 11.73 



Mean 11.94 



And 



'^ — 29.31, or barometer at lower station 700 -|- 29.31 = 729.31 millimetres. 



Delcros's tables, with these data, would give for the difference of level 349.76, 

 instead of 350 metres ; the corresponding error in the height of the barometrical 

 column does not exceed 0.08 millimetre, and thus remains within the limits of error 

 which may be expected in an ordinary observation. 



The principal object of this table, however, is to furnish the scientific traveller 

 with the means of readily computing on the spot approximate differences of level, 

 by sim|)ly multiplying the difference between the readings of the barometer at each 

 station by the half sum of the numbers in the table corresponding to the data given 

 iy the observations. 



Example. 



Suppose the barometer at the lower station to read 732.5, and at the upper station 

 703.2 millimetres ; the temperature of the air being respectively 18° and 16° Centi- 

 grade. 



The difference of the barometers, supposed to be reduced to the same temperature, 

 is 29.3 millimetres. 



Then, Table XIX. gives for 18° Centigrade and 73 centimetres, 11.73 

 " " for 16° Centigrade and 70 centimetres, 12.15 



Half sum, or mean, 11.94 



And, 29.3 X 11.94 = 349.8 metres = difference of level requn-ed. 



By the large tables of Delcros, we find for the same data 350.1 metres. 



This table can be considered as a complement to Delcros's tables, and mav save 

 the traveller the trouble of carrying the larger tables. 



A similar table in English measures is found above, at the end of the author's 

 larger tables (Table VI.), page 48 of this series, and another, more extensive one, 

 below, page 92, the use of which is explained by the examples just given. 



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