and Rate of a Chronometer. 411 
is then a matter of some difficulty to ascertain the time of con- 
tact fo a@ second. 
I have been compelled to pay particular attention to the prac- 
tice of the method of determining the time, from having often 
to depend upon it for the altitudes in lunar observations; as, 
it is seldom that the altitudes of the sun and the moon, or the 
moon and a star, can be got conveniently, at the same time, by 
reflection. 
In taking either altitudes or lunar distances I direct my as- 
sistant (who is generally one of the young men attending the 
school) to mark to the nearest second, the time when I give no- 
tice that the contact is perfect ; and from the nature of the ob- 
servations, I see no reason to believe that it can be of any ad- 
vantage to mark it nearer. 
The formula by which General B. has computed 'the time is 
that which furnishes the rule given in Prob. VI. Requisite Tables. 
There is a typographical mistake in the analytical statement of 
it ;—-sine (co. lat.+ declin.) ought to be sine (co. lat. + declin). 
This formula is not generally esteemed the most convenient one 
for computing the horary angle; but that is of little importance 
in the presence case, as it is correct. 
In taking altitudes for the time I do not always make the in- 
crements of altitude equal; but when the whole time of observa- 
tion is small, and the sun is at a considerable distance from the 
meridian, the increments of altitude are nearly proportional to 
the intervals of observation, so that any irregularity is easily de- 
tected. I then determine the error of the clock; not from the 
mean of the altitudes, but from the mean of the errors deduced 
from each altitude calculated separately, and this is exactly Ge- 
neral B.’s method. After I have finished my observations, 1 
generally make my young men take altitudes for the same pur- 
pose; and a comparison of their results with my own shows me 
at once whether their observations have been made with requisite 
correctness. 
I perfectly agree with General B. in the opinion which he ex- 
presses of the utility of the sextant, as it is now made by Trough- 
ton, Jones, and other respectable workmen. Jt is a portable 
handy instrument; it requires few adjustments, and those are 
easily made. Its state can be ascertained at almost any time ; and 
a little practice renders its use extremely easy. It is, in short, 
in many cases, a valuable substitute for more expensive aud com- 
plicated instruments. 
But with deference to General B., I would here suggest that 
the sextant can scarcely be considered as an instrument, the prin- 
ciples and the use of which are not generally well understood. If 
the 
