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252 «CT M. J. Nicklés on Friction and Pressure. 
ing all tended toa more perfect knowledge of the affinities of plants, 
we are assured have been the effect of progress in the right direction.” 
Without doubt the preponderating tendency of the ablest and 
most experienced botanists of the present day is, to cancel nominal 
species, and, taking a more eularged view of specific characters, 
to reduce slight or varying modifications to a common type; 4 
point not yet reached by zoologists, though probably it will be 
hereafter. Dr. Hooker’s tendency in this direction is evidently 
very decided ; possibly too much so. But it must be allowed 
_ that, while the botanists who multiply species unduly are always 
those who work upon scanty materials, or whose personal obser- 
vations are limited to a single district of country ; on the other 
hand, those who have access to the largest collections, or who 
have themselves botanized over various parts of the world, are for 
the most part as strongly inclined in the opposite direction. No 
tr. Hooker possesses both these advantages in an eminent de- - 
gree. Young as he still-is, no living botanist has investigated on 
the spot so many and so widely separated floras, and few like him 
have had constant access to the largest and best determined her- 
baria in the world. he principal danger here arises Irom 
Pembaras des richesses. It is hardly possible that a vast series of 
apparently confiuent forms should receive the detailed examina- 
tion which the less privileged botanist may concentrate upon his 
fewer materials ; and much is left to the quick, almost intuitive 
judgment, which is liable to error, indeed, but in which the true 
genius of a botanist is generally disclosed. 
rhaps there is no equally well known flora which compels 
the botanist to allow of such wide limits of variation in so large 
a proportion of its species, as that of New Zealand: at least $0 
it would appear at first sight. Yet this character seems to be 
exhibited more or less by insular floras generally, in which a con- 
siderable number of the peculiar species are apt to be surprisingly 
polymorphous, as was remarked by Bory de St. Vincent half @ 
century ago. =" 
(To be continued.) 
SS al 
Art. XXVII—On the relations which exist between Friction 
and Pressure; by M. J. Nicxtas. 
Iy preceding numbers of this Journal I have proposed for the 
Mechanical Arts the use of magnetic adhesion, a principle purely 
physical; and it becomes necessary to ascertain the relations 
which exist on a horizontal plane— : A 
tween pressure and the friction of iron sliding on iron. 
2. Between the pressure which one of these masses of iron 
converted into an electro-magnet exerts on the other, and the ef- 
