o do a great deal of
serviceable work even in this rough-and-ready way; but after a time I
grew dissatisfied with it--I wanted some means of measuring which should
be just as rapid but a great deal more accurate. I thought the matter
over for a long time, and at last hit upon the idea of turning the
telescope to account. The way I did it was this. You have, of course,
found that if you look through your telescope at an object, say, half a
mile away, and then direct the instrument to another object, say, four
miles off, you have to alter the focus of the glass before you can see
the second object distinctly. It was this peculiarity which I pressed
into my service as a means of measuring distances. My first step was to
secure a small, handy, but first-rate telescope--the best I could
procure for money; and, provided with this, I commenced operations by
looking through it at objects, the exact distances of which from me I
knew. I focused the glass upon them carefully, and then made a little
scratch on the tube showing how far it had been necessary to draw it out
in order to see the object distinctly; and then I marked the scratch
with the distance of the object. You see," pointing to the tube, "I
have a regular scale of distances here, from one hundred yards up to ten
miles; and these scratches, let me tell you, represent the expenditure
of a vast amount of time and labour. But they are worth it all. For
instance, I want to ascertain the distance of an object. I direct the
telescope toward it, focus the instrument carefully, and find that I can
see it most clearly when the tube is drawn out to, say, this distance,"
suiting the action to the word. "I then look at the scale scratched on
the tube, and find that it reads six thousand one hundred feet--which is
a few feet over one nautical mile. And thus I measure all my distances,
and am so enabled to make a really satisfactory little survey in a few
minutes as in the case of this afternoon. You must not suppose,
however, that I am able to measure in this way with absolute accuracy; I
am not; but I manage to get a very near approximation to it, near enough
for such purposes as the present. Thus, within the distance of a
quarter of a mile I have found that I can always measure within two feet
of the actual distance; beyond that and up to half a mile I can measure
within four feet of the actual distance; and so on up to ten miles,
which distance I can measure to with