THE HAMILTON ASSOCIATION. 17 
Now to make the axis, A. B., level. This is done by 
means of a striding level, a glass tube nearly filled with spirit 
of wine or sulphuric ether, being portion of a ring of large 
radius. The striding level of the transit instrument of the 
Toronto Observatory has a radius of about 760 feet. ‘The level 
is provided with means of making horizontal and vertical ad- 
justments. Each foot of the level has two planes inclined 
about 70 degrees and forming a V, so as to rest upon the points 
of the transit. The level should be so adjusted that its axis 
may be parallel with the axis A. B. of the transit. Place the 
level upon the pivots of the axis, and by turning the screw 
which lowers or raises the end of the axis, bring the bubble of 
the level to the centre of the glass tube. Then reverse the 
position of the level on the pivots of the axis, and if the bubble 
settles in the same position as before we can conclude the axis 
of the transit is horizontal, but, if the bubble does not return to 
the centre the amount of this difference will be equal to twice 
the inclination of the axis to the horizon. To remedy this turn 
the screw at the end of the axis so as to move the bubble over 
half this distance, and then raise or lower the vertical adjust- 
ment of the level until the bubble is in the centre of the tube. 
It is difficult to make this adjustment perfect at a single trial, 
and the operation must be repeated until the bubble occupies 
the same place in both direct and reversed positions of the level. 
When this is done satisfactorily the level will be in a plane 
parallel to the axis, A. B., but not necessarily parallel to one 
another. ‘Io determine whether such is the case, revolve the 
level slightly upon the axis of the transit. If the bubble 
changes its place the axis of the level must be inclined to the 
axis of the transit, and we must turn the screws of the horizon- 
tal adjustment for the level either backward or forward until 
a slight rotation of the level about the axis A. B. causes no sen- 
sible change in the position of the bubble. The former adjust- 
ment must now be verified again. 
Before proceeding with the second adjustment, that is the 
line of collimation, the wires have to be made perpendicular. 
Direct the telescope upon a small, well defined distant object, 
