A NEW TYPE OF SUNDIAL. 



691 



except those directly controlled by the Observatory. In this 

 instrument the axis points exactly north and south, but its socket 

 is loose enough to permit of rotating the plate. The stand, which 

 is exceedingly rigid, carries a projecting clamp on one side, which 

 enables the plate of the dial to be fixed after rotation, and, on 

 the other side, a small flat plate graduated into divisions, each 

 representing one minute of time on the dial. This is the " correct- 

 ing-plate,'" and is rigidly fixed into position so that its zero is 

 opposite a fixed point on the dial (I use 2 p.m. on the dial for 

 this purpose*) on those days of the year when the correction is 

 zero, and all sundials (in 28° E.) read correctly, vis.^ 23rd 

 September and 9th December. Finally, in the centre of the dial 

 there is inscribed a list of the corrections throughout the year, 

 and, owing to the slow variation of these corrections, only about 

 30 of these figures require to be engraved on the dial. The method 

 of using the dial is : ( i ) Find the nearest day to the date in this 

 list of corrections, and note the correction, say, -f- 7 minutes ; 

 (2) unclamp the dial and rotate it slightly until the 2 p.m. mark 

 is opposite + 7 on the correcting-plate, then reclamp the dial. The 

 time may then be read ofi: from the upper&dge of the shadow of the 

 axial wire where this shadow comes ofif the dial. Once adjusted, 

 the dial gives the correct time throiighout the day and for several 

 successive days, until the amount of the correction has changed ; 

 and, during those periods of the year when the correction is at 

 a maximum or minimum, it does not require readjustment for a 

 month. 



The following is the table of corrections for Johannesburg, 

 the correction for any place being equation of time plus (4 X 

 (30° — longitude)) : — 



