[ 56 o ] 
rectangular form, and the fpaces GHV remain for 
the title , and other infcriptions. 
VII. Another, and not the lead: confiderable, pro- 
perty of our map is, that it may, without fenfible 
error, be ufed as a fea-chart ; the rumb-lines on it 
being logarithmic fpirals to their common pole t y as 
is partly reprefented in the figure : and the arithme- 
tical folutions thence derived will be found as accu- 
rate as is necedary in the art of failing. 
Thus if it were required to find the courfe a fhip 
is to deer between two ports, whofe longitudes and 
latitudes are known, we may ufe the following 
Rule. 
j To the logarithm of the number oj minutes in the 
difference of longitude add the con ft ant logarithm * 
— 4. 1015 i 05, and to their fum the logarithm fine of 
the mean latitude , and let this laf J'um be S. 
' 'The cotangent of the mean latitude being T, and 
an arithmetical mean between half the difference of 
latitude and its tangent being called m, from the lo- 
garithm of T -|- m take the logarithm of T — m, 
and let the logarithm of their difference be D ; then 
fall S — D be nearly the logarithm tangent of the 
angle , in which the flip's courfe cuts the meridians . 
Note , We ought, in driftnefs, to ufe the ratio of 
+ to tx — .vR indead of ’T-fm to 
T — m\ but we fubditute this lad as more 
eafily computed, and very little different. 
* This conftant logarithm contains the reduction of the difF. 
of longitude to parts of radius unity, and to Briggs ' s Modulus. 
Example 
