272 Method of ascertaining Distances. 
gard to the bubble by which the line BC was ascertained ; and 
then the instrument is fit for use. For the visual ray at four 
chains distance points four feet higher on the staff, by the an- 
gular bubble being brought level, than it does by the horizontal 
one being brought so: consequently the variation of the obser- 
vations taken upon the staff, will be always more or less at any 
further or intermediate distance, as may be seen on reference to 
the figure. 
Operation.—If at any distance or station from the instrument 
the horizontal ray points to (say) 3°754 onthe staff; elevate the 
telescope as before until the angular bubble stands horizontal ; 
then if the visual ray poiuts to (say) 9°568, the difference is 5°S14, 
or five chains, 81 links and 4-10ths in distance, according to 
this adjustment, which I have preferred, because unity, or 1-000 
feet variation of the rays upon the staff, gives one chain in di- 
stance, and therefore 1-i0Q0dth of a foot gives one link,—and so 
on vice versd. 
Remarks.—It is necessary here to observe, that the staff FG 
has a sight or sliding vein with a nonius upon it, by means of 
which every foot is divided into 1-000 equal parts. But other 
staffs may be divided into feet and inches, or any other equal parts, 
and the angular bubble may be adjusted so that any number of 
inches or other parts on the staff shall be made to answer for any 
desired distance, at the convenience or discretion of the engineer. 
I presume it will be readily admitted by all engineers, that a di- 
stance of twenty miles may be levelled to within three inches, or 
less, of truth ; and it is even possible it may be performed cor- 
rectly true. Now, supposing, in taking levels across a country, 
the greatest probable error to take place, viz. three inches: then 
as this mode of ascertaining distances has only the same chance 
of varying from truth, as the operation of taking levels has, the 
greatest probable error that may occur, will only amount to one 
pole or five yards and a half in twenty miles. But if, as before 
stated, the art of levelling can be performed true (of which no 
reasonable doubt does exist), the distance can by this means be 
ascertained true also. It is much to he doubted if any engineer 
or surveyor has ever ineasured a distance of tweuty miles on the 
various inclined surface of the earth, to within twenty perches of 
truth, by the usual mode of chain dragging, subject as that me- 
thod is toso many unavoidable inaccuracies. Therefore, if truth 
can be approached. by this mode nearer than by any other zow 
used, in the proportion of 20 to 1, it cannot fail being an ob- 
ject of some consideration to the practitioner, and his employers 
in general.—This method was applied to practice in 1804, but 
not published. Joun Hugues. 
XLV. On 
