BAROMETRICAL TABLES. XXX1U 



The combined reduction of the mercury to the freezing point and of 

 the scale to 62° Fahrenheit brings the point of no correction to approxi- 

 mately 28?5 Fahrenheit. For temperatures above 28?5 Fahrenheit, the cor- 

 rection is subtractive, and for temperatures below 28?5 Fahrenheit, the 

 correction is additive, as indicated by the signs ( +) and ( — ) inserted 

 throughout the table. 



The table gives the corrections for every half degree Fahrenheit from 

 O to ioo°. The limits of pressure are 19 and 31.6 inches, the corrections 

 being computed for every half inch from 19 to 24 inches, and for every two- 

 tenths of an inch from 24 to 31.6 inches. 



Example : 



Observed height of barometer = 29.143 

 Attached thermometer, 54?5 F. 



Reduction for temperature = — 0.068 



Barometric reading corrected for temperature = 29.075 



TABLE 47. 



Table 47. Reduction of the barometer to standard temperature — Metric 

 measures. 



For the metric barometer the formula for reducing observed readings 

 to the standard temperature, o° C, becomes 



C = - B (m ~ J)t 



1 + mt 



in which C and B are expressed in millimeters and / in Centigrade degrees. 



m =0.0001818; / =0.0000184. 



In the table, the limits adopted for the pressure are 440 and 795 milli- 

 meters, the intervals being 10 millimeters between 440 and 600 millimeters, 

 and 5 millimeters between 600 and 795 millimeters. 



The limits adopted for the temperature are o° and + 35?8, the inter- 

 vals being o?5 and i-o from 440 to 560 millimeters, and o°2 from 560 to 

 795 millimeters. 



For temperatures above o° Centigrade the correction is negative, and 

 hence is to be subtracted from the observed readings. 



For temperatures below o° Centigrade the correction is positive, and 

 from o° C. down to — 20 C. the numerical values thereof, for ordinary baro- 

 metric work, do not materially differ from the values for the correspond- 

 ing temperatures above O C. Thus the correction for — 9 C. is numeri- 

 cally the same as for + 9 C. and is taken from the table. In physical work 

 of extreme precision, the numerical values given for positive temperatures 

 may be used for temperatures below o° C. by applying to them the follow- 

 ing corrections: 



