THE HAMILTON ASSOCIATION. 75 
latter kind is in use in the Toronto Observatory. In the grid- 
iron pendulum the rod is principally composed of a num- 
ber of parallel bars of steel and brass so connected together that 
the expansion of the steel bars tends to lower the bob of the 
pendulum, and the greater expansion of the brass to raise it, so 
that when the total lengths of the steel and brass bars are pro- 
perly adjusted a perfect compensation occurs, the centre of 
oscillation remaining at a constant distance from the point of 
oscillation. In the mercurial pendulum the weight which 
forms the bob is a cylindrical glass vessel nearly filled with 
mercury. With an increase of temperature the rod lengthens, 
but the mercury expanding must rise in the cylinder so that 
when the quantity of mercury is properly proportioned to the 
length of the rod the centre of oscillation, as with the gridiron 
pendulum, remains at the same distance from the point of oscil- 
lation. To clocks that are exposed to sudden changes of tem- 
perature, the gridiron is preferable to the mercurial pendulum, 
as the large body of mercury takes up the air temperature more 
slowly than the thin metal rods. 
All observatories, as a rule, have at least two clocks, one 
standard clock keeping sidereal time and one keeping mean 
solar time of the meridian in use. They are carefully mounted 
on stone piers, which are disconnected from the walls or floors, 
and are also protected as much as possible form sudden changes 
of temperature. 
Used in connection with both clocks is an electro chrono- 
graph, of which there are various forms, a description of which 
I will not here enter upon, as it would make my paper too 
lengthy, but suffice it to say that they are contrivances for re- 
cording the clock beats on paper with a pen or pencil held in an 
electro magnet. The paper is wound on a cylinder about seven 
inches or more in diameter, driven by clock work at a speed 
which will make the seconds marks on the paper about an inch 
apart, and thus enabling the smallest fractional portion of a 
second to be measured. The connection of the clock with the 
chronograph is made either with a break or a make circuit. 
The latter is in use in the observatory here on the sidereal clock, 
while both circuits are on the mean time clock. 
