The Determination of Longitude. 393 
bild, near Schaffhausen,' show an overlying deposit with 
“neolithic” implements and bones of recent animals, a bed 
of rubble and loam, destitute of human remains, and below 
this a bed containing bone implements, worked flints, and 
traces of cookery of the Palanthropic period. The whole 
rests on a bed of rolled pebbles supposed to be the upper 
part of the glacial deposits. This shows the interval be- 
tween the Palanthropic and Neanthropic periods, and also 
the Post-glacial date of man in Switzerland, 
THE DETERMINATION OF LONGITUDE.” 
By Professor C. H. McLrop. 
If we suppose the earth to be cut by 24 planes passing 
through the polar axis and being equally spaced, the surface 
traces will be along lines called meridians, and any point on 
one of these meridians will be 15 degrees or one hour in longi- 
tude distant from any point on the next adjoining meridian. 
They will be 15 degrees apart because the whole 360 de- 
grees has been divided by 24. Since the earth revolves on 
its axis once, or through 360 degrees, in 24 hours it will 
turn through 15 degreesin one hour. So that the longi- 
tude distance between the meridians may be spoken of in- 
differently as 15 degrees or one hour. We shall perhaps 
avoid difficulty if we speak of longitude differences in hours, 
or time, only. 
If, as a matter of convenience, we make one of our meri- 
dians pass through a given point such as Greenwich, then 
the longitude ofa point on another meridian may be de- 
scribed as one, two or three, etc., hours east or west of 
Greenwich, as the case may be. Thus we say approxi- 
metely, Montreal is in longitude five hours west of Green. 
wich, or more precisely, the longitude of the transit instru- 
ment in the Observatory is 4h. 54m. 18s., decimal something 
1 Nouvelles archives des Missions, &c., Vol. III., noticed in ‘‘ Natural Science,”’ 
1893. 
2 Somerville Lecture—delivered April 13th, 1893. 
