Solar Magnetic Period. 437 



has not entered, the final result as plotted on a curve will 

 match the mean magnetic curve by moving the latter about 

 eight hours to the right. 



Since the epoch is that of the mean ordinate corresponding 

 with noon, the real beginning of the magnetic curve is twelve 

 hours earlier. There is this practical advantage in retaining 

 the ephemeris on the noon ordinate, namely, that in following 

 the fractional part of the dates, this ordinate may oscillate 

 between the one and the ninety-nine hundredths of a day, and 

 yet without changing the name of the day when it exceeds 

 fifty hundredths. Therefore in tabulating data through many 

 periods the integer date of the day can be always taken and 

 the fraction neglected, since it is clear that the oscillation due 

 to fractions of a day will soon balance themselves. The con- 

 venience of this device in practical work is very evident. 



There are some plausible reasons for adopting the 115° merid- 

 ian west longitude as the prime meteorological meridian for 

 the United States. Near it is the junction of the high polar 

 circuit with the midlatitude circuit ; the low pressure belt is 

 thus chiefly interrupted by temperature waves at this point ; 

 near it is the greatest range in the temperature intensities ; 

 from it most conveniently can be computed the rate of the 

 eastward march ; the weather conditions originating on this 

 stage of the meteorological field are projjagated with such other 

 variations as are due to the relaxation and disintegration of the 

 same across the United States ; the convenience of a meridian 

 for meteorological reductions and the use of the ephemeris ; 

 the simple practical correction, half a day, from European to 

 American phenomena. One point should be carefully remem- 

 bered by meteorologists. It is necessary to distinguish between 

 the phenomena of the pure ether and of ponderable matter. 

 Magnetic forces are due to ether stresses alone, and hence those 

 produced by the solar field are cosmical, and act instantaneously 

 as well as simultaneously over the whole earth ; when this 

 ether energy is absorbed by the gases of the atmosphere, the 

 forces are converted into those having their seat in ponderable 

 matter, and hence the time factor due to convection or conduc- 

 tion must not be disregarded. Now the polar magnetic field 

 chiefly concentrates in British America, and this combined 

 with the general movements of the air causes the resulting 

 temperature effects to be felt in other places only as the motion 

 of the air allows them to spread. Hence at other stations, as 

 in the east of the United States, the time discrepancy between 

 the variations of the magnetic field and the temperature 

 changes differ by three or four days. It is necessary to add to 

 the epoch of the ephemeris this number to bring the synchro- 

 nism approximately to view. 



