THE SEA FISHERIES OF EASTERN NORTH AMERICA. 153 
Tlis treaty. is referred to in the Report of the British Sea Fisheries 
Commission, where it is expressly stated that the vessels of Belgium, 
with which there was no such treaty, were not bound by it, and that 
there was nothing to prevent. their fishing if they were so minded, in- 
dicating that the submission to a restriction must be a matter of joint 
agreement between two contracting parties (p. lxiv). 
With reference to the difficulty of estimating the extent of the three- 
mile limit, Prof. George F. Barker, writing from Brookfield Center, 
Conn., September 7, 1877, said: 
“ With reference to the question you propose, 7. e., Whether the proba- 
bility of an accurate judgment of distance is greater when the estimate 
is made by an observer standing on shore or by a person in the vessel, 
I would say that in my opinion the probability of a correct estimate of 
distance is considerably greater in the latter case. Distance, according 
to the present theory of vision, is always estimated by the eye from the 
magnitude of the visual angle under which the distant object is seen. 
Now, since any given object, placed at a suitable distance, will subtend 
any angle whatever, it is obvious that size and distance are both vari- 
ables in the calculation, and that if neither is given the problem is in- 
determinate. A man who does not know how large the object is which 
he sees, cannot, from this datum ‘alone, form any accurate idea of its 
distance. Hence, to estimate the distance of any object accurately, the 
size of the object which subtends the given visual angle must be accu- 
rately known. A man of average height placed a mile off will subtend 
an angle of about two minutes, and if two miles off, of about one minute. 
To tell that he is two miles off, and not one mile, the eye must accu- 
rately appreciate this slight difference of one minute of are. The hu- 
man height is so well known that persons are often introduced into art 
compositions to assist in judging of distances. But at three miles dis- 
tance, a man is too small an object by which to estimate distance by 
the unaided eye, the limit of error being so large as to render the esti- 
mate of no value. Hence, other familiar objects larger in size must be 
chosen. If a person on the shore, accustomed to this kind of estimate, 
sees a vessel which he is familiar with at the landing, he can tel! ap- 
- proximately her distance, if she is not too far off. Soa person sailing 
away from the shore may estimate quite accurately his distance from 
it, provided he be familiar with the size of the objects on shore. If 
neither person knows by personal inspection the size of the object looked 
at, the one in the vessel has the advantage, because the sizes of houses 
and their parts, windows, doors, &c., and also of well-known trees and 
animals, vary much less than the sizes of vessels. But there is another 
advantage on the side of the man in the vessel. He forms his judg- 
ment not by a comparison with a single object, but from a large num- 
her of objects, whose sizes are well known; and his estimate is, therefore, 
the mean of a large number of separate judgments, and so more reliable 
than any single one. Moreover, if these objects are successively back 
