115 
The appendages now described come into operation, when it is found 
that the dial shows incorrect time, notwithstanding that all its parts are 
in proper relation and condition ; for it is then certain that the proxi- 
mity of iron is the cause. Let the dial be brought to some unquestion- 
able situation in the room or elsewhere: it will take its just position. 
Draw the folding arms fully out. Turn the dial in such a manner as 
will cause the indicator to lie exactly over the index mark on the end 
of the magnetic bar; both indicator and index mark will then point to 
the degree on the declination are which corresponds with the declina- 
tion of the needle at that period. Note the degree of the graduated 
circle to which the compass-needle happens at that time to point; or if 
it do not point to any one exactly, move the arm a little, that it may do 
so, and tighten the joint. 
Tn this state let the instrument be brought to the place of sunshine, 
taking great care not to alter the relative position of the parts: it is to 
be placed in such a position that the compass-needle shall again poimt 
exactly to the same degree as it did previously. It will now be found 
that the dial has turned on its pivot several degrees, more or less, away 
from the indicator. Advance the iron ball to one side or the other of 
the magnetic bar, until the latter be attracted to its proper position, 
which will be when the index mark again coincides with the indicator. 
The dial will now show solar time correctly, notwithstanding the pre- 
sence of the mass of iron which caused the disturbance, and will remain 
in proper position, undeviatingly, while this condition of its parts 
continues. 
Nothing remains to be described except that on the lower part of the 
glass shade is a table of the equation of time for every day in the year. 
The following is a summary of the advantages expected from this 
kind of sundial, if accurately ‘constructed :—It may be removed from 
place to place, according to change of sunshine, when it will sponta- 
neously assume its proper position. Its situation, whether temporary 
or permanent, may be in immediate proximity to the iron fixtures of a 
house, yet no error of time will result. The very small error occa- 
sioned by variation of terrestrial magnetism is easily rectified when its 
amount is worth while. The reading of the shadow on the graduation 
is rendered more precise. The penumbra is practically effaced. The 
error of the common sundial, amounting to two minutes, during the 
passage of the sun across the meridian, and to one minute before and 
after that period, does not in this dial occur. Finally, it is obvious that 
although in point of fact the influence of diurnal magnetic declination 
is not avoided, its effect on the time indicated is altogether too minute 
to be appreciable on so small a dial. 
In conclusion, it is to be observed that, unless when the hour is re- 
quired to be known with great accuracy, as for setting clocks, nothing 
more than simple inspection of the shadow on the dial is necessary. 
A catalogue of Donations to the Library received since 9th November 
last was read, and thanks returned to the several donors. 
