EXTEMPORARY MEASUREMENTS. 
55 
The pace is commonly supposed to be 2i feet, but this is a most 
uncertain mode of measurement. Very few men, without practice , can 
take correctly a hundred consecutive steps or paces of the same length. 
Practice will determine the amount of ground covered in a certain 
number of paces, if tried over known distances ; it of course varies, but 
from experiment the mean has been found nearly as follows:— 
Pacing, at 30 inches per pace, of 108 in a minute, equals 270 feet, or 
3*068 statute, or 2 66 geographical miles per hour. 
Pacing quickly, at 30 inches per pace, of 120 in a minute, equals 
300 feet, or 3*41 statute, or 2*96 geographical miles per hour. 
Pacing slowly, at 36 inches, may average 60 per minute, equals 180 
feet, or 2*04 statute, or 1*78 geographical miles per hour. 
Distance by Sound . 
Sound travels at the rate of about 1090 feet iq. one second in calm 
weather and temperature 32° Fahr., and increases at the rate of 1 * 15 foot 
for each degree of temperature above 32°; a moderate breeze accelerates 
or retards sound by about 20 feet in a second. When a gun is used 
to measure distance it should always be pointed at an angle of about 
45° to the horizon. This method will be found most useful in making 
rough surveys of winding rivers or lakes, where it is impossible to land 
on account of the dense undergrowth or the swampy nature of the banks. 
Greater accuracy may be obtained if a gun is fired at each end. A base 
for a small triangulation can be measured by this means. 
Ascertaining Heights by Angles of Elevation . 
When using an angle of elevation to ascertain the difference of height 
of a mountain top and the position of the observer, it must be recollected 
that, if at any considerable distance, a large part of the mountain is below 
the horizontal line, and therefore the perpendicular of a right-angled 
triangle will only represent a portion of the height. To allow for this, the 
following correction, which includes mean refraction and curvature, must 
be added to the true angle of elevation. 
„ ... i _ distance in geog. miles x 100 
Correction, m seconds of arc, =—— - j -—— 
