52 Introduction: Migration. 
feet (whicli seems to be within what has actually been observed — Diet. B. 
p. 563) a hill of 2000 feet could be seen at a distance of 190 English miles, 
and similar heights in the land the bird is leaving would disappear at this 
distance behind.’) This of course could only take place under suitable atmo- 
spheric conditions, but, should such conditions be thought impossible, it may be 
mentioned that Meyer when. in Celebes was able from the top of Mt. Klabat 
(over 6000 feet) to see the islands of Maju and Tifore in the Molucca Straits, 
a distance of about 90 English miles, and the guide, the Hukum kadua of 
Ayermadidi, who had ascended the mountain four times, stated that Ternate could 
sometimes be seen, a distance of about 170 miles. The hills of Tifore are 
only 300 — 400 feet high, but the Peak of Ternate reaches 5300 feet. The 
effects of refraction may still further increase the length of view. It would 
be of interest to know whether the birds ever attempt their great journeys across 
the Pacific at night. As to nocturnal migration it should be remembered that 
nights are seldom of a “pitch” dark character, when we are not aware that 
birds ^) are heard migrating, and it is not unreasonable to suppose that they 
keep in view the outlines of the country over which they wander. As to the 
young birds migrating alone, it may be that a small percentage of adults among 
them may have been overlooked by those who have made these statements; 
like many others we do not entirely trust the evidence. But if it be accepted, 
such cases seem to point to “inherited experience”, the young bird recogmising 
the right way by innate knowledge, in the same manner as a newly hatched 
chick knows that grains of rice are good to eat. If birds have a “sense of 
direction”, why do they take such indirect routes to their destinations? 
*) Prof. Pattenhausen of the Royal Technical School in Dresden has most obligingly given ua the 
following formula for making this computation: Granted the atmosphere as perfectly transparent and, further, 
that the rays of light travel directly onward from the surface of the sea, the equation is: 
s = 3-S{yh+yh’) 
when h = the height of the point of view in metres, 
/<' = the height of the object seen in metres, 
and s = the radius of view in kilometres. 
2) Diurnal birds almost certainly have a long and clear range of vision, although it may not be fair to 
cite the Vulture, Kite, and Kestrel in proof At night they seem to experience much the same difficulty as 
ourselves. Those who have practised netting along hedgerows with a beater on one side and a double-handed 
clap-net on the other are well aware that it is next to no use attempting to catch birds except on dark nights, 
and even then the great majority see the net against the sky and avoid it. But the manner in which a scared 
party flutter in some neighbouring hedge to which they have flowm , is in part due to the difficulty they have 
in finding a perch in the dark, and show's that they do not see distinctly. 
I 
