64 General Notes, [January, 
ence of elevation of 200 feet. Where the slopes are gentle the 
curves are far apart, while among the cliffs of the mountains and 
plateaus, they are crowded together, in many places being almost 
run into a single line. 
These curves are not “run,” nor are they accurately located, as 
would be done in a minute survey. They do, however, express 
the orography, and, approximately, the elevation, over: the whole 
ma 
They are constructed mainly from the perspective sketches, 
aided and directed by the measured heights. As an example o 
the method of their construction, take a mountain spur, starting 
from the peak and extending to the valley below. Its summit 
and base, and each point of change of slope, are located, and their 
heights are known; we have also a profile sketch of the spur. 
Given these data, and what is easier than to distribute the curves 
with a considerable approach to accuracy, between those points 
actually determined. 
his method of representing orography is a strictly natural i 
one. Supposing the light to be vertical, grade curves must neces- 
sarily produce the same lights and shades as in nature. All the 
details of the topography, down to forms not above 200 feet in 
leight may be expressed. 
For masses for the representation of geological outcrops and 
formations, these grade curves are invaluable. They enable the 
geologist to draw accurately the outcrops not only of horizontal, 
but of inclined strata, over vast areas, from a few isolated obser- 
vations. In many cases, by thus expressing the orography of 
a range, the key to its geological structure is supplie 
he maps of this Survey have been justly regarded as among 
the finest specimens of cartography ever published in this or any 
other country. 
AFRICAN EXPLORATION.—Dr. Rohlf’s expedition to Wadai left 
Bengasi on the Fourth of July, last, for the Kufara oasis. The 
oasis of Djalo and Aujila were found to be ninety-eight and sixty 
feet respectively above the sea level. Heretofore they have been 
thought to be below it. At Kufara the party were attacked 
and plundered and obliged to return to Bengasi. 
hammedan life and become better fitted for longer journeys in 
Central Africa. The Academy states that Mr. Donald Mac- 
kenzie has returned to England from Cape Juby, in North-west- 
ern Africa, where, during his stay, many people came to enqui~e 
about trading and stated that there were numerous animals of all 
kinds in the interior, and the country was very fertile. Mean- 4 
Dr. Oscar — 
Lenz, weli known for his explorations on the Ogoowé, has been | 
sent by the German African Society to Morocco where he intends _ 
to cross the Atlas and investigate the geology and natural history 
of the southern districts. The Society also intends sending young _ 
travelers to this country, where they can get accustomed to Mo- _ 
