OBSERVATIONS FOR TIME AND LONGITUDE. 
199 
Almanac/ twice the sidereal time of the moon’s semidiameter passing 
the meridian (col. 7 “ Moon Culminating Stars/’ f Nautical Almanac ’), 
and the result will be the right ascension of the other limb. 
To the constant log 4*635480 (the log of 12 hours expressed in seconds) 
add the ar-co-log of arc A expressed in seconds, and the log of C; the 
sum of these three logs, rejecting 10 in the index, will be the log of 
approximate longitude in time, which call T'. 
Enter table No. XXII. with B at the top, and the approximate longi¬ 
tude in time, T', at the side, and find the corresponding correction, to 
the log of which add the constant log 4*635480 and the ar-co-log of A, 
and the sum, rejecting 10 in the index, will be the log of the correction 
to be applied to the approximate longitude in time with the same sign as 
B, and thus the correct value of T will be obtained, which will express 
the longitude of the place if it be west of Greenwich, but if the longitude 
is east we must subtract this value of T from 12 hours to obtain the 
true longitude in time east of Greenwich. 
In taking this observation it is essential that the axis on which the 
telescope turns be made horizontal. This is tested with the striding 
level, and the necessary correction obtained in the following manner. 
When the striding level is in perfect adjustment and placed on a truly 
horizontal axis of the instrument, the bubble will be in the centre of its 
run. Should this not be the case, and if with the level in perfect adjust¬ 
ment the bubble does not return to the centre of its run when reversed, 
the axis is not truly horizontal, and the inclination must be measured 
by the number of divisions. Place the striding level on the pivots and 
read the scale at the extremities of the air bubble. Reverse the bubble 
and again read the scale in the same manner; that is with the same end 
of the level on both east and west pivots alternately. This operation 
should be repeated several times in order to diminish the effect of 
incidental errors. Half the difference of the means of the readings will 
be the amount of the deviation. The maker should supply the value in 
arc of the divisions on the level, but should he neglect to do so the value 
may be obtained by placing the level lengthwise on the telescope and 
measuring the effect of changes of level on the graduated vertical arc. 
