264 1. P. Косн. 
about ten minutes is necessary; but this knowledge will be easily 
procurable by means of an examination of the determination of 
latitude immediately preceding. This little calculation occurs so fre- 
quently that it is of practical value to know how much 10 kilometres 
in east-west means in minutes. 
If we put 30 kilometres — 16 minutes we get on 80° 
16 il 
те 3 
ТЕ 21002 30° cos80° > 15 minutes (of time) 
== about 10 >< | = = — about 2 minutes. 
Let us presume that the sun during the preceding observation 
culminated at about 11755" and that since by the hodometer we 
have travelled 76 km towards WNW on about 80? latitude. The 
equation of time and the clock correction to Grw. is supposed to be 
unchanged in minutes from the last observation. 
The distance towards west then becomes 76 cos 221/2°. As cos x 
may now approximately be put = 1 + ©, 76 cos 22!/2 becomes nearly 
76-76 x (2) 3 = 76 + 76 x 555 — 76 + 76 x 15 — about 70km 
` 57 2 у т ; 12 
— about 14 minutes (of time). 
According to this the sun should culminate at about 11149. 
The little estimate of cos 22!/2° is after all superfluous. The error 
in the distance of the hodometer may easily become as great as 
10 kilometres. In case the hodometer indicates 76, and presuming 
one has followed a normally sinuous route, one might fittingly have 
estimated the distance towards west at 60 kilometres, the clock time 
of the culmination thus becoming about 11847. 
After this one ought to be ready to commence the observations 
at 11530™, especially if one has no assistant to record and look 
after the watch during the observation. The unpacking and setting 
up of the instrument generally takes five minutes; but one ought to 
reckon with a quarter of an hour. Not only is it good for the 
instrument to stand for about ten minutes in the sun, before the 
observation commences (as to this see under observations of the sun 
in the permanent Observatory, p. 93) but it also frequently happens 
that it takes longer to adjust the instrument, for instance because 
the bubble of the level turns out not to be of proper size. 
If one can get an assistant to record and read off the watch, it 
is of course a help; but at determinations of latitude one can very 
— 
1 1 10° 1 
1) — = —___. = about 1: = about 1:- = about 6. 
’ cos 80° sin 10° в > 6 u 
On this occasion as well as at several other estimates of a similar kind, a 
certain intimate knowledge of trigonometric functions is of practical importance, 
as one cannot always lay one’s hand on an engineer’s scale or a logarithm table. 
