56 
HINTS TO TRAVELLERS. 
Example .—-Observed with a theodolite the elevation of Kilimanjaro to bo 
6° 3' from a position afterwards found to be 25 miles distant, 
95 x 100 
Correction = = 625" = 10' 25" 
Corrected elevation = 6° 03' + 10' 25" - 6° 13' 25" 
Constant log. (of 6046 ft.).3 • 7815 
Log. tangent 6° 13' 25". 9*0376 
Log. 25 ......... 1*3979 
Height above observers position = 16,480.feet . log - 4*2170 
Flashing Signals. 
A flash from a small mirror is of the greatest use in surveying. Mirrors 
mounted so as to turn in any direction are sold by opticians under the 
name of heliostats, and a flash from one of two inches square may be seen 
fifty miles. It requires, however, an intelligent person to direct the 
mirror, and cannot therefore be worked by a native or untrained 
European. Mirrors fitted for this purpose are made of accurately parallel 
plate glass, and a small hole is made in the silvered surface and the 
plate protecting the back of the glass. 
Planting the stand of the mirror fairly, the hole in the centre is looked 
through, and a piece of paper working on a stick, which must be stuck in 
the ground about ten paces distant, is brought into exact line with the 
object to. which it is desired to flash and when the observer is in 
readiness to take, the angle to the flash.- The mirror is then turned about 
until the flash from the sun illuminates the paper, when the observer at 
the distant point will also see it. The flash must be kept carefully on the 
paper until an answering flash shows that it has been seen and observed. 
Two surveyors working together in this way can obtain most accurate 
observations without any time being expended in erecting marks. In a 
persistently cloudy climate, the method is, of course, of little use. 
Measurement of the Number of Cubic Feet of Water conveyed 
by a River in each Second. 
The data required are—the area of the river-section and the average 
velocity of the whole of the current. All that a traveller is likely to 
obtain, without special equipment, is the area of the river^section and the 
