the Pendulum vibrating Seconds in the Latitude of Loidun. V77 
a number of bisections of these dots gave their distance equal to 
36° 00016 inches of Sir George Shuckburgh’s scale. 
Meastrement of the Pendulum. 
_ ‘The pendulum was let into a solid piece of mahogany edge- 
wise, to such a depth that the knife edges were about one- 
twenticth of an inch above its surface. To one end of the pen- 
dulum acommou spring steelyard was attached by its hook, and 
a string being passed through the ring, and fastened to an up- 
right piece of wood screwed to the end of the mahogany case, 
the pendulum was extended by a force rather greater than its own 
_ weight (about ten pounds), and consequently, no error (if any 
such were to be apprehended) could arise from a difference in the 
length of the pendulum in its vertical and horizontal positions, 
The knife edges were fixed as nearly as could be done by me- 
chanical means, at right angles to the bar of the pendulum ; but 
the bar being flexible, they would most probably, when the pen- 
dulum was extended for the purpose of measurement, be found 
to be not precisely parallel to each other, and would consequently 
require some adjustment. To effect this, two opposite screws 
were passed through the sides of the mahogany case, so as to 
act in a transverse direction against that extremity of the pen- 
dulum which was nest the steely ard; and the microscopes being 
brought over the extreme points of the knife edges, alternately 
on either side of the bar, the requisite parallelism was readily 
obtained by means of the screws, suflicient room pavies heen 
left in the mahogany case for the very small motion of the ex- 
tremity of the pendulum which might be found necessary. ‘This 
arrangement is represented in’Plate III. fig. 5.* 
To obtain the distance between the knife ec dges, two different 
methods were used. For the first, four rectangular pieces of 
brass were prepared, about half aninch square. Very near to the 
perfectly straight edge of each, a fine line was drawn, to be 
viewed through the microscope, and these lines were each crossed 
at right angles by two others, intended to indicate that part of 
possibly be occasioned by an error in the divisions bounding that part of 
General Roy's seale which I had employed, I compared it with various other 
portions, and found no greater difference than might haye been expected 
trom unavoidable imperfection of division, It is to be presumed then, that 
the error into which Sir George Shuckburgh appears to have fallen, must 
haye arisen from the two scales not having been of the same temperatur e 
at the time they were compared, particularly as Sir George Shuckburgh’s 
is by far the most massive of the two. I may here ‘add, that last winter 
wishing to know whether the expansion of the two scales was equal, 
roughly compared them together once, at the temperature of 33°, when 
appeared that 42 inches on General Roy’s scale, was equal to about 42:00] 
inches of Sir George Shuckburgh’s standard. 
. ® [This Plate will be given with a future Number.) 
Vol. 52, No.245, Sept. 1818. M the 
