THE WELDON RANGE-FINDER. 
349 
attached with a lever-hitch, and each yard is marked by a little scrap 
of red (or other) wool sewn on by fine thread. Practically, until we 
get an old line, we do not add the red marks, it would be too much 
trouble to alter them often; the parchment marks are easily altered. 
These marks are numbered from 1 upwards. Each represents 100yds. 
of range—thus" Y*T~ -'reads 2000yds. C) reads 1500yds., 
and each red (gm mark reads ( f5) 20yds. The 
strap is to put V —s over the pole I v s am about to de¬ 
scribe. I am particular in describing each thing used as, since we 
began, we have been constantly altering, and nothing has been done 
without a reason generally too long to describe. 
At least two strong poles should be made (three are better) some¬ 
what of the following dimensions, light and strong. The iron shoe and 
END 
" zziizs \ ® 
i 
^ _ __ _ _ ^ _ __ 
rim enables the stick to be fixed in the ground readily, using the foot 
on the rim. In the end of the stick a hole is bored just large enough 
to receive the handle of an instrument. 
The screws on the glasses are to enable the instruments to be 
adjusted. This requires a little knack. At this place we have a care¬ 
fully measured base/with points marked over which the glasses are 
placed, and it takes a very short time to adjust errors. Once adjusted 
correctly they very seldom vary, even with very rough handling, but 
it is found that a first adjustment of anew instrument is seldom correct, 
for the screws give a little. I doubt if you will find the glasses I send 
appreciably out, but change of climate may affect them, and we have 
so little change of temperature here that I have no experience on that 
point. I should be slow to alter them. If both agree exactly it is 
unlikely they have altered much, and, unless the eye is practised, it is 
apt to imagine its own faults are those of the glasses. The easiest way 
to get a base of adjustment is to use a theodolite, taking every care to 
adjust it properly. Take a known distance from a good map (say from 
one of the vanes on the Academy to some marked point on the Barrack 
Field), set off an Z of 88° 34' 3", measure along the base ^th of the 
known distance. On each of the points set up perpendicularly one of 
the sticks, and on each stick place an instrument, then in each instru¬ 
ment (if in adjustment) will be seen the image of the distant object 
coinciding with the centre of the opposite instrument. If out of 
adjustment, take out with a small screw-driver the outer screw of the 
brass guard which covers the three adjusting screws, and adjust the 
angle of the glasses by loosening and tightening the centre and two 
outer screws alternately, but taking care to leave them all tight. I 
have said there is a knack in doing this, and if not properly done it 
will be found that a little rough usage, such as dropping the instru¬ 
ment a few times, will alter the adjustment. When well adjusted it is 
not very easy to put the glasses out. 
43 
