430 Dr. Lamont on the Ten-year Period which exhibits itself 



year, and the others under the name of the summer half-year. In 

 this way we ohtain the following table : — 



A p-lance at this table is sufficient to show that a periodical 

 increase and decrease of the magnitude of the daily motion takes 

 place. Applying the same method as that made use of by Sir 

 John Herschel in similar cases, I have drawn a curve, which by 

 regular curvature passes as nearly as possible through the ends 

 oAhe single ordinates, and find 1843-5 to be the epoch of the 

 minimum, and 1848-5 the epoch of the maximum. 



Shortly before the commencement of the series of observations, 

 a maximum must have taken place. To ascertain the epoch of 

 this maximum, we will call in the aid of the Gottingen observa- 

 tions. From the Resultaten des Magnetischen Vereins we deduce 

 the following numbers as expressing the magnitude of the motion 

 for the single years. 



Making use of the method above alluded to, I derive from these 

 numbers that the maximum took place in 1837-5. 



If we go still further back, the observations at present exist- 

 ing do not furnish us with any data until we reach 1820. In 

 this year the series of observations undertaken by Col. Beaufoy 

 at Bushy Heath was finished, and from these we are enabled to 

 make further determinations. The magnitude of the motion for 



