Paa et Dampskib, der gjør omkring 10 Mils Fart 
(v = 5), vil det altsaa være heldigt at observere Vindret- 
ningen paa Søen, naar Vindstyrken er under 1—2 Beau- 
fort Scala. Man faar den da paa 1 Stregs Nøjagtighed, 
og nøjere kan vanskelig svage Vindes Retning bestemmes. 
Disse Grænser svare til den Fejl, man kan erholde i Vin- 
kelen u med en Vindmaaler, der maaler Vindhastigheden 
med en Nøjagtighed af 1” pr. Sec. Paa Dampskibe, som 
gjøre en stor Fart, over 16 Kvartmil i Timen (8” pr. Sec.), 
maa Retningen af Vinde indtil 3 Beaufort Scala obser- 
veres paa Sgen. 
Vi have endnu et Tilfælde, i hvilket det er nødven- 
digt at observere Vinkelen w. Det er, naar Vindens Ha- 
stighed ombord er mindre end Vindmaalerens Frictions- 
coefficient. I dette Tilfælde bevæger Vindmaaleren sig 
ikke, og Størrelsen w kan ikke maales. Til at bestemme 
W har man da alene Formelen 
v=vDV. 
sin (U—7) 
Efter denne Formel kan man finde den sande Vind- 
hastighed uden Vindmaaler, alene ved Loggen og observe- 
rede Vindretninger. Formelen giver imidlertid i enkelte 
Tilfælder, som naar og w er 0, ikke noget Resultat. 
De følgende Betragtninger gjælde kun det Tilfælde, 
at den ombord følte Vindhastighed er mindre end Vind- 
maalerens Frictionscoetfieient, et Tilfælde som indtreeffer, 
naar Vinden kommer agtenfra og dens Hastighed er meget 
nær saa stor som Skibets Fart, eller naar Vind- 
hastighed og Skibets Fart ere meget smaa. Kaldes Fric- 
tionscoefficienten f, er altsaa Betingelsen, at w < /. 
baade 
Differentieres Ligningen for IV, faar man: 
sin 4y 
CY sin (u—y) $ 
lv + 
22 
sin % ] 
=== Gl 
sin? (u—y) Y 
With a steamer running 10 miles an hour (v = 5), 
it will therefore be best to observe the direction of the 
wind on the sea-surface when the force of the wind is less 
than 1—2 Beaufort Scale. The accuracy attainable can be 
within a point, and more accurately the direction of a faint 
breeze can hardly be determined. These limits correspond 
to the error that can vitiate the determination of the angle 
u with an anemometer that measures the velocity of the 
wind with an accuracy of 1 metre per second. With 
steamers of great speed, running upwards of 16 miles an 
hour (8 metres per second), the direction of the wind up 
to 3 Beaufort Scale must be observed direct from the 
sea-surface. 
There is another case in which it is necessary to 
observe the angle w, viz. when the velocity of the wind as 
felt on board is less than the friction-coefficient of the 
anemometer. In that case the anemometer does not revolve, 
and the value of w cannot be measured. The only way of 
determining W is then by means of the formula — 
veg ee 
sin (w—y) 
By this formula the true velocity of the wind may 
be found without the anemometer, from the log alone and 
the observed directions of the wind. In some cases, how- 
ever, as, for instance, when y and w = 0, this formula 
gives no result. 
The following remarks apply only to the case in which 
the velocity of the wind as felt on board is less than the 
friction-coefficient of the anemometer, a case that occurs 
when the wind comes right aft and its velocity is very 
nearly as great as the speed of the vessel, or when the 
velocity of the wind and the speed of the vessel are both 
very low. Hence, if we call the friction-coefficient f, w is 
assumed to be < /. 
By differentiating the equation for W, we get: — 
cos (u—y) 
= — du. 
sin? (w—y) 
—v. sin ¥ 
sin ¥ 
dw 
Il. 
dv 
= , dw sin ¥ 
age 7 = 0 aller 1800 ee == = RA = I Ij, 
dv ap Sim 9) oe 
Samtidig er y = 0. Fejlen i Loggen gaar over i 
Vindhastigheden. Er u = 90°, hvilket kun kan finde Sted, 
å paso OW sag Å 
naar v < f, bliver pp Ga tang y. Den største 
Værdi af tang y bliver VEE = VEN en Stor- 
J 
relse, der nærmer sig ~, naar v nærmer sig til 0. 
Sør . 
Den 
snu) 
OMY Sm 
dv  + sm y 
In that case y = 0. The error m the log passes 
wholly into the velocity of the wind. If w = 90°, which 
Wihen w= 0; ox 180% ieee 
n occu > GY sin, 9 Å 
can occur only when v < f, als aaa dam. he 
= 2 
greatest value of tany will be vipa oe 
when v approximates 0. 
J 
a magnitude that tends towards 
