<s>
The	O
Drift	B-Device
Sight	I-Device
was	O
a	O
bombsight	B-Application
developed	O
by	O
Harry	O
Wimperis	O
in	O
1916	O
for	O
the	O
Royal	O
Naval	O
Air	O
Service	O
(	O
RNAS	O
)	O
.	O
</s>
<s>
The	O
Drift	B-Device
Sight	I-Device
eliminated	O
the	O
need	O
for	O
a	O
stopwatch	O
to	O
perform	O
this	O
calculation	O
,	O
as	O
on	O
earlier	O
devices	O
,	O
and	O
greatly	O
eased	O
the	O
bomb	O
aimer	O
's	O
workload	O
.	O
</s>
<s>
The	O
Drift	B-Device
Sight	I-Device
was	O
quickly	O
introduced	O
into	O
RNAS	O
service	O
and	O
quickly	O
thereafter	O
by	O
the	O
Royal	O
Flying	O
Corps	O
(	O
RFC	O
)	O
as	O
well	O
.	O
</s>
<s>
In	O
British	O
service	O
,	O
Wimperis	O
 '	O
Course	B-Device
Setting	I-Device
Bomb	I-Device
Sight	I-Device
(	O
CSBS	B-Device
)	O
started	O
replacing	O
the	O
Drift	B-Device
Sight	I-Device
in	O
1917	O
,	O
but	O
it	O
remained	O
in	O
widespread	O
use	O
in	O
the	O
US	O
Army	O
Air	O
Service	O
into	O
the	O
1920s	O
.	O
</s>
<s>
In	O
US	O
use	O
the	O
Drift	B-Device
Sight	I-Device
is	O
often	O
referred	O
to	O
as	O
the	O
Wimperis	O
sight	O
,	O
but	O
this	O
name	O
is	O
more	O
commonly	O
applied	O
to	O
the	O
CSBS	B-Device
,	O
especially	O
in	O
Commonwealth	O
air	O
forces	O
.	O
</s>
<s>
Prior	O
to	O
the	O
introduction	O
of	O
the	O
Drift	B-Device
Sight	I-Device
,	O
bombsights	B-Application
were	O
generally	O
very	O
simple	O
systems	O
of	O
very	O
limited	O
accuracy	O
.	O
</s>
<s>
The	O
Central	O
Flying	O
School	O
bombsight	B-Application
(	O
CFS	O
)	O
replaced	O
this	O
in	O
1915	O
,	O
but	O
was	O
difficult	O
to	O
install	O
in	O
the	O
cockpit	O
.	O
</s>
<s>
In	O
1916	O
,	O
Henry	O
Wimperis	O
started	O
design	O
of	O
a	O
new	O
bombsight	B-Application
,	O
working	O
in	O
collaboration	O
with	O
Scarff	O
.	O
</s>
<s>
His	O
"	O
Drift	B-Device
Sight	I-Device
"	O
included	O
a	O
simple	O
system	O
for	O
calculating	O
the	O
effects	O
of	O
wind	O
.	O
</s>
<s>
This	O
measurement	O
was	O
taken	O
prior	O
to	O
the	O
bomb	O
run	O
,	O
using	O
a	O
secondary	O
sighting	O
system	O
at	O
the	O
rear	O
of	O
the	O
main	O
bombsight	B-Application
.	O
</s>
<s>
This	O
adjustment	O
also	O
automatically	O
moved	O
sights	O
at	O
the	O
front	O
of	O
the	O
bombsight	B-Application
fore	O
and	O
aft	O
,	O
directly	O
setting	O
the	O
bombsight	B-Application
to	O
calculate	O
the	O
drift	O
of	O
the	O
bombs	O
due	O
to	O
the	O
measured	O
wind	O
.	O
</s>
<s>
The	O
original	O
design	O
was	O
only	O
suitable	O
for	O
low-level	O
use	O
and	O
was	O
later	O
known	O
as	O
the	O
Low	O
Height	O
Drift	B-Device
Sight	I-Device
Mk	O
.	O
</s>
<s>
Although	O
the	O
Drift	B-Device
Sight	I-Device
was	O
a	O
significant	O
improvement	O
over	O
earlier	O
designs	O
,	O
it	O
still	O
required	O
the	O
aircraft	O
to	O
fly	O
up	O
or	O
downwind	O
on	O
the	O
final	O
bomb	O
run	O
.	O
</s>
<s>
Over	O
land	O
,	O
as	O
anti-aircraft	O
guns	O
grew	O
more	O
proficient	O
,	O
this	O
became	O
a	O
serious	O
concern	O
as	O
they	O
would	O
pre-sight	O
for	O
firing	O
along	O
the	O
wind	O
line	O
,	O
and	O
using	O
the	O
bombsight	B-Application
under	O
fire	O
was	O
difficult	O
.	O
</s>
<s>
There	O
were	O
instructions	O
on	O
how	O
to	O
use	O
the	O
Drift	B-Device
Sight	I-Device
for	O
cross-wind	O
bombing	O
,	O
but	O
this	O
was	O
complex	O
and	O
apparently	O
rarely	O
used	O
.	O
</s>
<s>
The	O
Drift	B-Device
Sight	I-Device
was	O
introduced	O
in	O
1916	O
,	O
and	O
the	O
simplicity	O
of	O
the	O
device	O
in	O
terms	O
of	O
manufacture	O
and	O
mounting	O
to	O
the	O
aircraft	O
allowed	O
it	O
to	O
quickly	O
equip	O
RNAS	O
forces	O
.	O
</s>
<s>
By	O
1917	O
it	O
was	O
in	O
widespread	O
use	O
in	O
the	O
RNAS	O
,	O
and	O
was	O
selected	O
for	O
the	O
Handley	O
Page	O
O/400	O
bomber	B-Application
in	O
RFC	O
service	O
as	O
well	O
.	O
</s>
<s>
However	O
,	O
Wimperis	O
was	O
well	O
aware	O
of	O
the	O
shortcomings	O
of	O
the	O
Drift	B-Device
Sight	I-Device
in	O
terms	O
of	O
flying	O
along	O
the	O
wind	O
line	O
,	O
and	O
was	O
already	O
testing	O
his	O
solution	O
,	O
the	O
Course	B-Device
Setting	I-Device
Bomb	I-Device
Sight	I-Device
.	O
</s>
<s>
The	O
CSBS	B-Device
was	O
only	O
slightly	O
more	O
complex	O
than	O
the	O
Drift	B-Device
Sight	I-Device
to	O
build	O
,	O
adding	O
a	O
compass	O
and	O
another	O
adjustment	O
to	O
the	O
sights	O
to	O
account	O
for	O
cross-drift	O
.	O
</s>
<s>
Hundreds	O
were	O
in	O
use	O
by	O
1918	O
,	O
and	O
had	O
supplanted	O
the	O
Drift	B-Device
Sight	I-Device
by	O
the	O
war	O
's	O
end	O
.	O
</s>
<s>
A	O
major	O
difference	O
between	O
the	O
Drift	B-Device
Sight	I-Device
and	O
CSBS	B-Device
was	O
that	O
the	O
latter	O
needed	O
to	O
sight	O
below	O
the	O
aircraft	O
,	O
and	O
was	O
not	O
as	O
suitable	O
for	O
use	O
on	O
the	O
side	O
of	O
the	O
fuselage	O
.	O
</s>
<s>
IA	O
in	O
US	O
Army	O
Air	O
Service	O
use	O
for	O
some	O
time	O
,	O
as	O
they	O
did	O
not	O
have	O
dedicated	O
bomber	B-Application
aircraft	O
until	O
after	O
the	O
war	O
.	O
</s>
<s>
Oddly	O
,	O
when	O
development	O
of	O
new	O
bombsights	B-Application
started	O
in	O
the	O
1920s	O
,	O
these	O
were	O
based	O
on	O
the	O
Drift	B-Device
Sight	I-Device
design	O
,	O
not	O
the	O
CSBS	B-Device
.	O
</s>
<s>
III	O
design	O
from	O
the	O
CSBS	B-Device
,	O
and	O
it	O
was	O
borrowed	O
Mk	O
.	O
</s>
<s>
The	O
Drift	B-Device
Sight	I-Device
was	O
mounted	O
to	O
the	O
side	O
of	O
the	O
aircraft	O
using	O
two	O
brackets	O
at	O
the	O
front	O
and	O
back	O
of	O
the	O
sights	O
.	O
</s>
<s>
This	O
mounting	O
system	O
was	O
common	O
among	O
early	O
British	O
bombsights	B-Application
,	O
notably	O
the	O
Equal	O
Distance	O
Sight	O
that	O
led	O
to	O
the	O
Drift	B-Device
Sight	I-Device
.	O
</s>
<s>
In	O
the	O
Equal	O
Distance	O
design	O
,	O
this	O
movement	O
was	O
itself	O
the	O
main	O
aiming	O
mechanism	O
,	O
and	O
could	O
not	O
be	O
adjusted	O
for	O
ease	O
of	O
use	O
as	O
it	O
was	O
in	O
the	O
Drift	B-Device
Sight	I-Device
.	O
</s>
<s>
Sighting	O
was	O
accomplished	O
in	O
the	O
manner	O
of	O
iron	O
sights	O
on	O
rifles	O
,	O
and	O
the	O
Drift	B-Device
Sight	I-Device
used	O
the	O
same	O
terminology	O
of	O
"	O
foresight	O
"	O
and	O
"	O
backsight	O
"	O
,	O
although	O
"	O
lower	O
"	O
and	O
"	O
upper	O
"	O
would	O
be	O
more	O
accurate	O
physically	O
.	O
</s>
<s>
In	O
this	O
respect	O
the	O
Drift	B-Device
Sight	I-Device
was	O
similar	O
to	O
earlier	O
bombsight	B-Application
designs	O
.	O
</s>
<s>
As	O
the	O
Drift	B-Device
Sight	I-Device
only	O
worked	O
correctly	O
if	O
flying	O
straight	O
and	O
level	O
,	O
the	O
bombsight	B-Application
also	O
includes	O
two	O
spirit	O
levels	O
.	O
</s>
