and on the Dip of the magnetic Needle . 391 
give the mean monthly true variation, and mean monthly 
diurnal alteration of variation only; and they were deter- 
mined from a mean of the observations made at those times 
of the day when the variation was considered least, and 
greatest ; which variations for each month, may generally be 
considered as a mean of 600 observations. 
From the observations made by the late Dr. Heberden and 
others, about the year 1775, the variation was found to in- 
crease annually nearly io', since that time to the present, its 
rate of increase has been considered as gradually diminishing,* 
and for the last three or four years the alteration has been so 
very small as to make it somewhat doubtful whether it may 
* An exception to the progressive increase appears between the years 1790 and 
1 791, as the observations between these two years make it to decrease 2 or 3', and 
subsequent observations to increase again ; to what this should be attributed, I am 
at a loss to account, unless it arose from the alteration which took place in the iron 
work of the room in December, 1790; four strong iron braces having been applied 
to the girders in the floor of the great room of the Royal Academy, (which is 
over the Society’s meeting-room,} in consequence of a cracking noise made from the 
great pressure of a number of persons in the room during the time that Sir Joshua 
Reynolds was delivering a lecture; these braces were applied two on each side of, 
and equidistant from, the compass, the nearest, about 18 feet from it. It may be 
proper to mention, however, that haviug been favoured with the variation observed 
both by Mr. Cavendish and Dr. Heberden, in the above mentioned years the 
alteration of the variation was by the former nearly the same as in my own, but by 
those of the latter greater in both cases. 
An alteration took place between the observations made with the dipping needle 
in the same years. All the iron braces were on the north west side of the needle, and 
the nearest about 18 feet from it. 
The allowances made to the observations of the variation, and also of the dip, for the 
effect of the iron work of the room, were both ascertained after the above mentioned 
alteration in the iron work took place, but they have, notwithstanding, been applied 
to the observations made before as well as since that time. 
3E 
MDCCCVI. 
