In previous editions I reprinted here, with a few trifling alterations,
part of a paper that I originally communicated to the Royal Geographical
Society, and which will be found at the end of their volume for 1854. In
addition to it, communications are published there from Lieutenant Raper,
Admiral FitzRoy, Admiral Smyth, Admiral Beechey, and Colonel Sykes; the
whole of which was collected under the title of 'Hints to Travellers;'
they were printed in a separate form and widely circulated. When the
edition was exhausted, a fresh Committee was appointed by the Council of
the Royal Geographical society, consisting of Admiral sir George Back,
Admiral R. Collinson, and myself, to revise the pamphlet thoroughly. This
process was again gone through in 1871, and now the pamphlet is so much
amended and enlarged that I should do no good by making extracts. It is
much better that intending travellers should apply for this third edition
of the 'Hints to Travellers' at the society's rooms, 1, Savile Row: for
it gives a great deal of information upon instruments that they would
find of real value. Its price is 1s.
Porters for delicate Instruments.--Entrust surveying instruments and
fragile articles to come respectable old savage, whose infirmities compel
him to walk steadily. He will be delighted at the prospect of picking up
a living by such easy service.
Measuring low angles by reflexion.--an ordinary artificial horizon is
useless for very low angles. They can be measured to within two or three
minutes, by means of a vertical point of reference obtained in the
following manner:--Tie two pieces of thread, crossing each other at two
feet above the ground, put the vessel of mercury underneath it, and look
down upon the mercury. When the eye is so placed, that the crossed
threads exactly cover their reflexion, the line of sight is truly
vertical; and, if the distant object be brought down to them by the
sextant, the angle read off will be 90 degrees + altitude. Captain
George's arrangement of glass floating on mercury (made by Cary, Fleet
Street, London), allows of very low angles being observed, but the use of
this instrument requires considerable caution as to the purity of the
mercury and the cleanliness of the glass.
Substitute for glass roof to Horizon.--For want of a glass roof to place
over the mercury a piece of gauze stretched over the vessel will answer
very tolerably for the purpose of keeping off the wind. The diameter of
the pupil of the eye is so large, compared to the thickness of the
threads of the gauze, that the latter offer little impediment to a clear
view of the image.
Silvering Glasses for Sextants.--"Before taking leave of this subject it
may not be unimportant to describe the operation of silvering the glasses
of sextants, as those employed on surveying duties very frequently have
to perform the operation.
"The requisites are clean tinfoil and mercury (a hare's foot is
handy)--lay the tinfoil, which should exceed the surface of the glass by
a quarter of an inch on each side, on a smooth surface (the back of a
book), rub it out smooth with the finger, add a bubble of mercury, about
the size of a small shot, which rub gently over the tinfoil until it
spreads itself and shows a silvered surface, gently add sufficient
mercury to cover the leaf so that its surface is fluid. Prepare a slip of
paper the size of the tinfoil. Take the glass in the left hand,
previously well cleaned, and the paper in the right. Brush the surface of
the mercury gently to free it from dross. Lay the paper on the mercury,
and the glass on it. Pressing gently on the glass, withdraw the paper.
turn the glass on its face, and leave it on an inclined plane to allow
the mercury to flow off, which is accelerated by laying a strip of
tinfoil as a conductor to its lower edge. The edges may, after twelve
hours' rest, be removed. In twenty-four hours give it a coat of varnish,
made from spirits of wine and red sealing-wax. It may be as well to
practise on small bits of common glass, which will soon prove the degree
of perfection which the operator has attained." (Admiral Sir E. Belcher.)