Medens saaledes i 1877 og 1878 Skibets Hastighed i 
Observationsøjeblikket kunde bestemmes med al ønskelig 
Nøjagtighed, var dette mmgenlunde Tilfældet i 1876. Saa 
ofte der loggedes med den almindelige Log paa samme Tid 
som de meteorologiske Observationer toges, erholdt man den 
forønskede Nøjagtighed i Bestemmelsen af Skibets Fart. 
Men denne Log benyttedes forholdsvis sjelden. Patent- 
loggen, der viser udløben Distance, giver kun under jevn 
Fart et Maal for Hastigheden 1 Øjeblikket. Man var saa- 
ledes ofte henvist til at bedømme Skibets Fart ved Skjøn, 
og søgte at corrigere dette ved en senere Aflæsning af: 
Patentloggen eller ved de paa astronomisk Vej fundne Posi- 
tioner. Fejlen i Bestemmelsen af Skibets Fart kan saa- 
ledes vistnok i enkelte Tilfælder gaa op til 1 til 2 Kvart- 
mil i Timen eller 0.5 til 1 Meter 1 Secundet og derved 
blive af samme Orden som Usikkerheden ved de samme 
Aar med Vindmaaleren bestemte Vindhastigheder. 
Naar Skibet bevæger sig gjennem Luften, virker dets 
Bevægelse som en Modvind, der sammen med Luftens virke- 
lige Bevægelse har en Resultant, som er den Luftbevægelse, 
der føles og maales ombord. Beregningen af den sande 
Vindretning og sande Vindhastighed kan gjøres paa følgende 
Maade. 
Kaldes: 
y den observerede Vinkel mellem Vindretningen ombord 
og Diametralplanet. eller den observerede Vindretning 
p- C. ombord minus Kurs p. C. 
v Skibets Fart, regnet i modsat Retning af dets Bevægelse 
(Kurs), 
w den ombord med Vindmaaler maalte Vindhastighed, 
uw den virkelige Vindretnings Vinkel med Diametralplanet, 
den virkelige Vindhastighed. 
e Vinkelen mellem den observerede og den virkelige Vind- 
retning, 
saa har man Ligningerne 
WY so = w sin y (1) 
W cosu = W Cos Y—V (2) 
w cos Y—V 
cotang w = —— et (3) 
; w sin y 
W= Vu? + —2wveosy (4) 
Naar y = o, det er Vinden ombord foles ret forind, 
har man 2 Tilfælder: 
1° y = 0, wu = 0, det er, den virkelige Vind stik i Stevn, 
da er 
W = w—v 
2D =O UW = 80 der 
ind, da er 
W = v—w (6) 
Naar y = 1809, det er, Vinden ombord føles ret 
agterind, har man kun 1 Tilfælde, nemlig: 
(5) 
er den virkelige Vind ret agter- 
16 
Thus, whereas in 1877 and 1878 the rate of the ship 
could be determined with all desirable accuracy at the mo- 
ment of observation, such was by no means the case in 1876. 
Whenever the common log was used and meteorological 
observations taken simultaneously. we could obtain the re- 
quired accuracy tor determining the speed of the vessel; 
but this log was used comparatively seldom. The patent 
log, which shows the distance run, does not give a measure 
of the rate at the moment of observation, unless the 
speed be uniform. Hence, we had often to estimate the 
rate of the vessel, and then try to correct the result by 
a subsequent reading of the patent log or by means of 
astronomically found positions. The error in the determi- 
nation of the ship’s speed can, for 1876, therefore. in some 
cases amount to as much as 1 to 2 miles an hour, or 0.5 
to 1 metre per second, and accordingly rise to the same 
order as the inaccuracy attaching to the velocity of the 
wind determined the same year with the anemometer. 
The motion of a ship moving through the air acts 
as a head wind, which, together with the true motion of 
the air, produces as resultant the motion of the air as felt 
and measured at the place of observation on board. ‘The 
computation of the true direction and velocity of the wind 
can be effected in the following manner: — 
If 
y be the observed angle subtending between the direction 
of the wind on board and the fore and aft line, or the 
observed direction of the wind on board p. C. minus 
the course p. C.; 
v the speed of the ship taken in the opposite direction of 
her motion (course); 
velocity of the wind on board as measured with 
anemometer ; 
angle subtending between the true direction of the 
wind and the fore and aft line; 
w the 
the 
u the 
W the true velocity of the wind; 
e the angle subtending between the observed and the true 
direction of the wind; 
then we have the following equations: 
Wsinu = w sin y (1) 
Weost = W COS Y—V (2) 
W COS Y—V 
cotang % = —— ee (3) 
; w sin y 
W= Vw? + 7? —2wveosy (4) 
If y = 0, i.e. the wind on board is felt to blow right 
ahead, we have 2 cases: — | 
1° y = 0, w = 0, i.e. the true wind comes right ahead, 
and then 
W = w—v; 
2° y = 0, wu = 180°, i. e., the true wind comes right aft, 
and then | . 
W = v—w. 
If y = 180°, i.e., the wind on board is felt to blow 
right aft, we have only I case, viz.: — 
