80 
HilSTS TO TRAVELLERS. 
however, be of groat assistance to the traveller if he provides himself tvith 
a blank map, on the scale of ten geographical miles to an inch, of sufficient 
range in latitude and longitude to include the country he intends to 
explore. He should also procure some paper ruled with dark lines into 
inch squares, and then again subdivided into five smaller squares; this 
will be useful to him for plotting his work in the field, and should be 
made up in the form of an ordinary sketching-block. Should the lati¬ 
tude and longitude of the point of departure be known, the latitude and 
longitude of any place on his route can be approximately determined 
by working the traverse. It must not, however, be supposed that an 
accurate survey of a large tract of country can be made with the aneroid, 
prismatic compass, and boiling-point thermometer; the most that a 
traveller could expect to do with the aid of these instruments would 
be to make a rough sketch of the country through which he passed. 
But instances are not wanting where travellers, by a judicious use 
of these simple instruments, have added very considerably to our 
geographical knowledge. The map of ScliweinfurtlTs journey to the 
Welle is an example of what can be done with the material furnished 
by such observations. 
The weak points in this method of surveying are, the errors caused by 
false estimates of the distance travelled, and those arising from the effects 
of local attraction on the compass. Knowing these sources of error, 
every care should be taken to guard against them. With regard to dis¬ 
tance, the only safe way of estimating it is, by carefully noting the time 
occupied in passing from one place to another. In almost all countries 
bodies of men have a nearly uniform rate of progression, and by taking 
an early opportunity of noting this rate, the distance traversed in 
a known period of time can be fairly estimated. Schweinfurth, before 
setting out on his great journey to the Welle, carefully noted the time 
which it took him to pass over a known distance at a regular pace, to 
which he had trained himself; and truly wonderful results have been 
attained by native surveyors in India by following the same plan. The 
only precautions that can be taken against the effects of local attraction 
on the compass are, to be careful when taking a bearing to put all arms, 
such as rifles, at some distance from the compass; as a general rule, 
where possible, to avoid all rocks; and to take bearings both forward 
and backward on the route travelled, taking their mean as the magnetic 
