111 
MONDAY, JANUARY 10, 1859. 
James HentHorn Topp, D.D., President, in the Chair. 
His Grace the Duke of Manchester, Alphonse Gages, Esq., and James 
Graham Hildige, Esq., were elected Members of the Academy. 
Micwart Donovan, M. R. D.S., Hon. Member of the Philadelphia 
College of Pharmacy, &c., read the following paper :— 
DESCRIPTION OF A HORIZONTAL SUNDIAL, WHICH, WHEN REMOVED FROM ONE 
SITUATION TO ANOTHER, RESUMES THE POSITION NECESSARY FOR INDI- 
CATING SOLAR TIME; AFFORDS MEANS OF READING ITS INDICATIONS WITH 
PRECISION ; EFFACES PENUMBRA, AND REMEDIES CERTAIN LOCAL CAUSES 
OF IRREGULARITY. 
A sUNDIAL possessing the properties indicated by the title of this com- 
munication renders a little more complication necessary than has been 
hitherto resorted to in the construction of those generally simple instru- 
ments. The complication is to no great amount ; and, when the advan- 
tages are considered, it will perhaps appear that it was not introduced 
in vain. If accurately made, the instrument will do its duty with great 
exactness, will obviate certain local sources of error incidental to all 
movable dials, and with less trouble in the management than the de- 
scription of the mode of using it might lead a person to suppose. Most 
of the advantages of the dial are peculiar to itself, the means of obtain- 
ing them never having been applied to any other, although contributing 
much to its convenience, utility, and adequacy. The following brief 
account will sufficiently explain the construction :— 
The principle of the mariner’s compass affords the obvious means of 
obtaining the horizontal and meridional position of the dial. The dial- 
plate, of sufficient area to afford an open graduation, must be made of a 
very light, yet refractory material: such a substance is talc, a mineral 
unalterable under any circumstances of heat or moisture, even when as 
thin as paper. A circular very thin lamina of this mineral, about six 
inches in diameter, isto be covered on both surfaces with the thinnest white 
paper, on one of which the usual hour-lines with their subdivisions are 
to be drawn. This plate is to be mounted on a bar-magnet as a diameter, 
about the thickness ofa silver threepence, and about and half an inch in 
breadth, and in length a quarter of an inch longer than the diameter of 
the plate: its ends, therefore, project one-eighth of an inch beyond the 
circumference. A mark on the south end projection acts as the index 
to a graduated are of 30 degrees, drawn on a part of the circumference 
at the vacant south-east of the dial-plate. This is the declination arc. 
The index is made to correspond with such degree of the are as repre- 
sents the angle of magnetic declination of that period. According as the 
magnetic declination of any period is known to have varied, a corre- 
sponding variation must be made in the angle, by turning the dial-plate 
on the common axis of itself and the magnetic bar, until the index and 
R. I, A, PROC.—YVOL. VII. 8 
