liv INTRODUCTION. 



Table 79 gives the quantities to be subtracted from the hours, minutes 

 and seconds of a sidereal interval to obtain the corresponding mean time 

 interval, and Table 80 gives the quantities to be added to the hours, minutes 

 and seconds of a mean time interval to obtain the corresponding sidereal 

 interval. The correction for seconds is sensibly the same for either a sidereal 

 or a mean time interv^al and is therefore given but once, thus forming a part 

 of each table. 



Examples : 



Change 14^ 25'" 36f2 sidereal time into mean solar time. 



Corresponding mean time = 1423 14.4 



2. Change 13*' 37"^ 22^7 mean solar time into sidereal time. 



Given mean time == 13^ 37"^ 22^7 



Correction for 12^ = + 2°^ 8?i3 



37™ = + 6.08 



22^7 = + 0.06 



+ 2 14.27 +2 14.3 



Corresponding sidereal time = 13 39 37-o 



MISCEI.I.ANEOUS TABLES. 



DENSITY OF AIR AT DIFFERENT TEMPERATURES, HUMIDITIES AND 



PRESSURES. 



The following tables (81 to 86) give the factors for computing the 

 density of air at different temperatures, humidities and pressures. 



The formula from which they have been computed is, in metric 

 measures, 



g ^ 0.00129305 [7. 1 116153] / ^ — o.378g \ 

 I + 0.00367 / \ 760 / 



in which 8 is the weight of a cubic centimetre of dry air expressed in 

 grammes, under the standard value of gravity at latitude 45° 

 and sea level. 

 b is the barometric pressure in millimetres. 

 e is the pressure of aqueous vapor in millimetres. 

 / is the temperature in Centigrade degrees. 

 For dry atmospheric air (containing 0.0004 of its weight of carbonic 

 acid) at a pressure of 760 mm. and temperature 0° C, the absolute density, 



