$ 
HINTS TO TRAVELLERS. 
aperture, by a good maker. It should be mounted on a split 
tripod, and furnished with a Kelner eye-piece, of not less magni¬ 
fying-power than 40, and should be fitted with an arrangement 
by which it can, when removed from the stand, be screwed firmly 
to a tree or other support. The telescope should be tried on 
Jupiter, and found to give a satisfactory view of the satellites, 
before it is taken. 
* 
Plane table. — Two plane tables, and spare horse-hair for sight vanes. 
They should be in strong canvas bags with leather-covered 
corners, and furnished with straps, so that they can be carried 
like a knapsack. For information as to use and form of con¬ 
struction, see pp. 40, 42, and 97 to 109. 
Pedometer.— Apt to get out of order. If employed, at least three persons 
should each carry one. 
Clinometer. 
Pocket level (Abney’s), with a mirror to show where the bubble is when 
it is held to the eye. It also serves as a clinometer for the 
measurement of slopes. 
Pain gauge, see Vol. II., pp. 23 to 26. 
Examination of Instruments . 
Let every instrument be tested, and its errors determined and tabulated 
at the National Physical Laboratory, Bichmond, Surrey.* This is done 
for moderate fees. The following are some of the present charges:— 
Watches, A class, £1 Is., B class, 10s. 6d. ; ordinary thermometers, Is.; 
boiling-point thermometers, 2s. 6d.; marine and portable barometers, 
10s. 6d.; prismatic compasses, A class, 6s., B class, 4s. 6d. ; theodolites, 
* This should be attended to by the traveller, especially in the case of ther¬ 
mometers which have been previously examined at Kew Observatory, as it has 
been found that their errors change considerably; for instance, a boiling-point 
thermometer which was tested in 1881 was found, in five years, to have increased 
its error at some readings by no less than *2 of a degree, and in no part of the 
scale by less than * 1 of a degree. 
