262 
NATURE 
DD 
=<) 
[| fuly 1886 
tents throughout life, while the tracheides with bordered 
pits (transfusion tissue) contain, in the mature condition, 
nothing but water. 
On p. 164 the statement that there is finally “free 
communication” between the contiguous tracheides of 
the wood of Pinus is erroneous. The pits are closed, at 
any rate as long as the wood serves its main function of 
conveying the sap. 
At p. 171, in the same chapter, there is a repetition of 
Hofmeister’s old mistake as to the deliquescence of the 
original cell-walls of the endosperm in the Conifers. 
Strasburger showed in his ‘“‘ Angiospermen und Gymno- 
spermen,” that this idea was due to Hofmeister having 
confused the disorganised cells of the nucellus with those 
of the endosperm. The Conifers have one and the same 
endosperm throughout the development of the ovule : 
there is no distinction of “primary and secondary” 
endosperm. 
Judging from the footnote on p. 209, there seems to be 
some confusion between the xylem and the bundle-sheath 
in Trillium. 
It is to be regretted that the student is not shown how 
to investigate the minute structure of the angiospermous 
embryo-sac when ready for fertilisation. 
In spite of the rather serious faults noticed, the book 
on the whole is a good and useful one. De HES. 
MR. MERRIFIELD’S “TREATISE ON 
NAUTICAL ASTRONOMY” 
A Treatise on Nautical Astronomy for the Use of 
Students. By John Merrifield, LL.D., F.R.A.S. 
(London ; Sampson Low, Marston, Searle, and Riving- 
ton, 1886.) 
“THIS is an excellent work for the student, evidently 
compiled with considerable care, which may also 
be consulted with advantage by the seaman. Of course 
the author does not claim originality, excepting in one 
particular, viz. a method of his own for “clearing the 
lunar distance,” as, in point of fact, nearly everything the 
work contains has been published in previous treatises, 
Mr. Merrifield deserves, however, the credit of placing 
clearly before the student many points which are only 
touched on by other writers—-notably the account of the 
correction for refraction, and the explanation of the fact 
that the maximum altitude is not invariably the meridian 
altitude, a point which is only touched on by a footnote 
in Raper, and is usually ignored entirely; yet which is of 
considerable importance in the case of the moon. The 
examples, also, which are given at the end of each chapter 
are of great use to the student, as from them a knowledge 
is obtained of the subjects he is likely to be examined in, 
and as these questions have been selected from many 
examination papers, they are an excellent guide. In the 
theoretical part of nautical astronomy the book is nearly 
all that can be desired, and this part can always be learnt 
better on shore than in a ship, where the constant noise 
and interruption, together with perpetual motion at sea, 
renders study all but impracticable : in one or two cases, 
however, Mr. Merrifield also touches on the practical use 
of instruments, &c., and on these subjects he is naturally 
not so good an authority. 
It may perhaps, therefore, be 
advisable to point out the usual course of proceedings in 
Her Majesty’s surveying-vessels, both in correcting in- 
struments and also in ascertaining positions at sea. 
First, with regard to the sextant, the error of collima- 
tion is not readily obtained, as stars only are available, 
and there are no means of illuminating the wires in the 
telescope, so that a bright moonlight night is requisite. 
Secondly, with respect to the errors of centering and 
graduation, Mr. Merrifield suggests that the combined 
error should be ascertained by means of measuring the 
distance between several pairs of stars by the instruments, 
the correct distances having been previously calculated. 
But here the varying nature of the refraction prevents 
good results, and a better method is to measure the dis- 
tances both by the sextant and by the repeating circle, as 
in the latter instrument all errors are eliminated. 
In the account of the artificial horizon Mr. Merrifield 
says that “it is used for taking altitudes when the sea 
horizon is obscured,” being apparently under the impres- 
sion that it can be used on board a vessel. Were such 
the case, it would often relieve the mind of many an 
anxious navigator, but, unfortunately, the constant motion 
of a ship altogether precludes its use at sea; it is true 
that the late Capt. Becher, R.N., invented a method of 
observing altitudes at sea, in foggy weather, by attaching 
a small pendulum, suspended in oil, outside the horizon- 
glass of a sextant ; to this a horizontal arm was fastened 
which carried at its inner end a slip of metal showing the 
true horizon when seen in a certain position; but this 
did not prove a success, and is now almost forgotten ; 
and there is nothing to trust to but the compass and log 
when the horizon is obscured. ‘The true use of the arti- 
ficial horizon is to obtain observations on shore, and the 
sea horizon should never be used then. The best artificial 
horizon is a trough filled with mercury, covered with a 
glass roof, but this cannot be used in the extreme cold of 
the Arctic regions, and consequently there a plate of 
dark glass is substituted, which is adjusted by spirit 
levels. The error of the artificial horizon is due to two 
causes, first the imperfections in the glass roof, which, as 
Mr. Merrifield remarks, may be guarded against by 
reversing the roof ; and secondly, owing to the attraction of 
mountain masses causing the mercury to depart from the 
true level. Could some means be found which would 
enable the seaman to take observations, in a vessel, inde- 
pendently of the sea horizon, it would be the most useful 
nautical discovery of the age, but this is not to be effected, 
as Mr. Merrifield suggests, by mounting the artificial 
horizon on gimbals, for even if the ship were in herself 
rigid, the motion at sea would preclude the possi- 
bility of obtaining observations, as the position of the 
observers could not be changed with sufficient rapidity to 
suit the ever-varying angle of reflection from the horizon, 
with respect to the observer on the deck ; and Mr. Merri- 
field’s own experiences of the difficulties of obtaining ob- 
servations from the roof of a quiet house must have 
taught him that it would be much more difficult in a 
vessel which is constantly vibrating from the motion of 
the engines or other disturbing causes. The idea of 
placing a piece of glass on the mercury to still its vibra- 
tions, was some years ago promulgated by the late Staff- 
Commander George, attached to the Geographical So- 
ciety, who invented a very useful little artificial horizon 
PR a Boag a 
