212 COMPARISON OF THE SAROMETRICAL SCALES. 



the scale of the Metrical Barometer alone will actually represent the standard length. 

 and the millimeters will have the true length ; while the inches and lines of the Old 

 French and of the English Barometers will be too short, causing thus the barometrical 

 column to appear too high. If the temperature of the instruments be 62° Fahrenheit, 

 the divisions of the English Barometer will have the true standard length, and those 

 of the Old French Barometer nearly so ; but the millimeters of the Metrical Barome- 

 ter will be too long, causing the barometrical column to appear too low. It is to 

 neutralize the effect of those inequalities arising from the expansion of the scale 

 that it is necessary, before comparing the observations taken with the three barome- 

 ters, to reduce them to the same temperature. This is done by means of the tables 

 above mentioned, for reducing the barometer to the freezing point, which suppose 

 the scales to be of brass from top to bottom, and which take into account the expan- 

 sion or contraction they undergo by the variations of temperature. 



But in doing so, we must be aware that the accuracy of the comparison depends 

 in part upon the correctness of the indications of the attached thermometers, which 

 determine the amount of the correction to be applied for reducing the barometers to 

 the freezing point. If the thermometers do not agree, an error is introduced which will 

 affect the height of the reduced columns, and the final comparison. Therefore the 

 correction of the attached thermometers ought to be ascertained and applied to tnem 

 before the reduction is made ; or if this correction is unknown, it will be well to place 

 the instruments to be compared in the most favorable conditions for taking the same 

 temperature, and then to take the temperature given by one of the thermometers to 

 reduce both barometers. If the correction of the attached thermometer has not been 

 applied before the reduction, it will be contained, after the reduction, in the total cor- 

 rection of the instrument. If it be so, this circumstance must be indicated. 



In computing the following tables, the value of the Metre, as determined by Capt. 

 Kater, (Philosoph. Transact, for 1818, p. 109, and Baily's Astronomical Tables, p. 

 192,) has been adopted, viz. 1 Metre, at 0° Centigrade = 39.87079 English inches, 

 at 62° Fahrenheit. The relation of the Metre (legal) to the Old French system of 

 measures is known to be 1 Metre =; 443.296 French or Paris lines. From these 

 equations are derived the elements used in the computations, which are found at the 

 head of each table. 



Besides the larger Tables I.- VIII., a set of smaller ones, Tables IX. — XVI., has 

 been added, which will be found useful for comparing Barometrical differences, such 

 as ranges, amount of variation in a given time, &,c., expressed in measures of differ- 

 ent scales, in which only small quantities occur that are not found in the large tables. 



