290 
NATURE 
[ Fan. 25, 1883 
of the aspirant in surveying that over accuracy (that is 
such minuteness as cannot be represented on paper) is 
loss of time. 
The eighth chapter is on Triangulation, and is more 
worthy of attention than those preceding. If we remem- 
ber rightly, there was only one three-feet theodolite used 
in the Ordnance Survey of Great Britain, which instru- 
ment is the property of the Royal Society. In fact, so far 
as we are acquainted, there are only two three-feet theodo- 
lites in existence, Ramsden’s, and another used in the 
Great Trigonometrical Survey of India. 
The observation that very distant stations are generally 
observed at night is now subject to correction, as the 
heliostadt has rendered it quite as easy to observe by day, 
in fact, in some of our marine surveys, triangles whose sides 
were 60 miles in length, have been obtained with these 
instruments, and an eight-inch theo Jolite, with the greatest 
ease. With the eight-inch theodolite, and by means of 
repeating the main angles round the circle, very accurate 
results may be obtained; and the spherical excess has 
to be allowed for, and deducted, in order to make the 
triangles plane, for in all nautical surveys the chord of 
the arc is used both for calculation and plotting. 
The ninth chapter, on Levelling, contains not only an 
account of levelling, but also of obtaining heights by 
means of the barometer and thermometer, but totally neg- 
lects the method in general use by surveyors, viz. by 
angles of elevation and depression with a theodolite. For 
travellers the barometer and thermometer give an ap- 
proximation of the elevation, which is exceedingly useful 
in an unsurveyed district. For work requiring extreme 
accuracy careful levelling is required, but for nautical 
work the principal use of the levelis to ascertain the exact 
difference between the zero of the tide gauge and some 
permanent mark on shore, so that a fixed datum can 
always be referred to for reduction of soundings in future. 
The heights of hills are almost invariably obtained by 
angles of elevation and depression, and the results so 
closely approximate to the truth that it is waste of time to 
do more, unless results less than five or six feet in error 
are absolutely requisite. 
The tenth chapter is on Tides, and is, as has been 
before mentioned, well worthy of perusal, in fact, it is the 
most complete popular description we remember to have 
seen; and if compiled entirely by Mr. Robinson from the 
books he has consulted reflects great credit on him, and 
we can but wish he had paid the same attention to the 
other parts of his treatise. 
We think, if we remember rightly, that it was in one of 
the Arctic voyages, that of Sir John Ross, that the in- 
fluence of atmospheric pressure on the rise of the tide 
was first observed, but the fact is well known now, and is 
always allowed for by surveyors when ascertaining the 
mean level of the sea. This subject is of considerable 
practical importance, for it is sometimes the only guide 
we possess by means of which we can reduce our sound- 
ings to the same depth as those obtained at previous 
epochs. For instance, if the datum-mark to which the 
soundings have been referred, has, in the course of time, 
disappeared, the surveyor’s first work is to ascertain the 
height on his gauges of the mean level of the sea. This 
he does by obtaining day and night observations for five 
or six consecutive high and low waters, carefully register- 
ing the barometer at the same time. Then meaning 
the results and allowing for the atmospheric pressure, it 
is astonishing how closely they agree. The mean level 
having been found, it is very easy to reduce the soundings 
to the former datum. For instance, if the soundings are 
reduced to low water ordinary springs, and their rise at 
springs is 16 feet, it is evident that the soundings must be 
reduced to 8 feet below mean level of the sea, to enable 
them to be compared with those previously obtained. 
In his paragraph (201) on tide-gauges, Mr. Robinson 
recommends a string from a float over a pulley. It must 
be either a chain or wire, as a string is far too subject to 
contraction and expansion from atmospheric changes. 
Self-registering tide-gauges are, we are glad to say, 
becoming much more common than they were, and we 
trust to see them established at every important point in 
the United Kingdom. 
The eleventh chapter, on Soundings, may do very well 
to enable a sub-lieutenant to answer some questions in the 
examination-room, but is useless in practice. 
Few surveyors think it necessary to accurately protract 
on a chart the position of the objects they use for sounding 
transits (Art. 210). Often the back mark is too far off 
to appear at all on the sheet, and the farther off it is the 
better, provided the atmosphere is clear; for a front 
mark the first conspicuous object in the foreground is 
seized—a conspicuous tree, the chimney of a house, the 
angle of a hedge, a boat hauled up on the beach, &c. If 
the back object is sufficiently far off, the lines of sound- 
ings are practically parallel, and the same mark may be 
used for the whole survey. It is also requisite to cross 
the lines of soundings, to avoid any chance of error. 
The sounding on the chart depends (1) on the leadsman, 
(2) on the officer fixing and registering, (3) on the tidal 
register, and (4) on the reductions being correctly made. 
Now there may be a mistake in either one of these, and 
consequently it is advisable to always cross the lines of 
soundings asacheck. We have found that as good a 
method of checking the correctness of the results as any, 
is by running along the contour lines, as defined by 
soundings obtained at right angles to those lines. 
Another remark on soundings we must also take ex- 
ception to, viz. that (Art. 213) it is usual to make a reduc- 
tion of a couple of feet below low water in doubtful 
cases. 
With respect to under-currents (Art. 221), Mr. Robin- 
son appears not to be aware of the methods pursued by 
Sir G. Nares in the Challenger, and Capt. Wharton in 
the Shearwater. 
Chapters twelve and thirteen, on Chronometers, and 
Meridian Distances, are principally derived from Admiral 
Shadwell’s notes on the management of chronometers, 
andhere we recommend the student to the original rather 
than the copy. 
The method of calculating meridian distances ex- 
pounded by Dr. Tiarks, and fully explained by Sir 
Chas. Shadwell, is invariably used by nautical surveyors, 
and the results thus obtained have hitherto closely 
approximated to the later determinations by means of the 
electric telegraph. 
Chapter fourteen, on the method of Plotting a Survey, 
deals almost exclusively with the small plans required by 
the examination papers at the Naval College, and as it 
