58 
HINTS TO TRAVELLERS, 
By protracting the data on the first two lines, on ruled paper as 
described above, it will be found that the area of the section is 3260 feet, 
or thereabouts; this, multiplied by 150, gives 489,000 cubic feet of 
water as the contents of the river at any given moment between the line 
of soundings and the assistant. As this amount passes by in 38*4 seconds, 
the number of cubic feet per second is the former number divided by the 
latter, which gives 12,734. 
It must be distinctly understood that this number is only roughly 
approximate, and that it is excessive. However, with the above data, 
an engineer would be able to make a somewhat better calculation. In 
the meanwhile, the traveller might consider the flow of the river in 
question to be between 10,000 and 13,000 feet per second. 
Map Projections, 
Mercator's Projection . 
On a sheet of cartridge paper, 13 inches by 20, it is proposed to con¬ 
struct a map on Mercator’s projection, on a scale of 10 geographical miles 
to an inch equatorial— i.e< 6 inches to the degree of longitude. 
t • • i ,n ( Lat. 31° to 33° N. 
* \ Long. 34° to 36° E. 
Draw a base line, find its centre, and erect a perpendicular to the top of 
the paper; the extremes of longitude 34° and 36° added together and 
divided by 2, give 35°, the central meridian, and which is represented by 
the perpendicular; on each side of it lay off 6 inches, and erect perpen¬ 
diculars for the meridians 34 and 36; divide the base line into 10 geo¬ 
graphical mile divisions, and the part from 35° 50' to 36° 00' into 
geographical miles for the latitude scale. 
From Table A, take the following quantities 
Lat. 31° to 32° = 1° 10'*4 = the distance between parallels 31° and 32° 
„ 32° to 33° = 1° ll'T „ „ „ 32° „ 33° 
2°21'*5 „ „ „ 31° „ 33° 
Having thus obtained the distances between the required parallels, 
divide the map into squares of 10' each way, and the map is ready for 
the projection of the route. 
