EARTH'S MAGNETISM TO THE SOLAR AND LUNAR PERIODS. 101 



mum and minimum do not exist ; the periods, however, are variable, and in this 

 way they seem to balance each other in the mean ; the maximum of one day 

 occurring at tlie time of the minimum of another. (See Cm-ve, No. 1.) It will be 

 remarked also, with regard to the mean of the summer months, that the descend- 

 ing branch, after the G p.m. maximum, has a strongly marked concavity, indi- 

 cating a tendency to a minimum which does not decidedly shew itself. 



5. It has been shewn by Drs Lamont and Lloyd, that the morning maximum 

 seems to occur throughout the year a little before sunrise, and the afternoon maxi- 

 mum a little before sunset. I would mention another coincidence ; the times of 

 maximum atmospheric pressure and minimum intensity are the same through- 

 out the year, and also the times of minimum atmospheric pressure and maxi- 

 mum intensity. There is also a secondary minimum of pressure occurring about 

 the same hour as the secondary maximum of intensitj^ and the secondary maxi- 

 mum of pressure occurs about 10 p.m., the period of an inflexion in the intensity 

 cm've. 



6. The diurnal range of intensity is least in January, and greatest in July, 

 being 0000641 in January (the whole horizontal force being unity), and 0003396 in 

 July, or five times greater in July than January ; the mean diurnal range for the 

 year is 0*002041, being almost exactly the mean of the ranges for January and 

 July, and the same as the ranges for March and October. I have projected the 

 mean range for each month in 1844, and also the mean altitude of the sun for 

 each month. (See Curve, No. 2, Plate III.) There is a considerable similarity in 

 the forms of the two curves ; a marked inflexion occurs in June in the range curve. 

 As June is remarkably free from irregularities, it seems to me pi'obable that the 

 range for June is the true diurnal range freed from those irregularities termed 

 disturbances. If so, Ave may perhaps consider the deviation of the ranges for 

 other months from the curve of altitudes as due to disturbances ; in which case 

 we might conclude that disturbances increased the diurnal range most in Ai^ril 

 and October, and least in February and June. This, however, requires other proof 



7. At Makerstoim, in 1 844, each degree of the sun's altitude was equivalent 

 to a diurnal range of about 000006 of the horizontal intensity. 



8. The mean intensity at midnight and at 1'' p.m. are each equal to the mean 

 of the year, and the means at these hours for each month differ very little from 

 the mean of the month. This leads me to the monthly means, and their relation 

 to the period of a year. 



9. In June 1845, I shewed to the Physical Section of the British Association, 

 that the Makerstoun observations of horizontal intensity for 1842 indicated 

 well-marked double maxima and minima in the course of the year ; and that 

 as this result had been obtained from the means of only four observations in each 

 day, I had determined the temperature correction approximately for the Toronto 

 bifilar magnet, by the method already referred to, and applied this correction to 



