U. S. Coast Survey Reports—1861 and 1862. 99 
A very interesting part of the discussion of the Girard College 
observations is that which shows the influence o 
the magnetic horizontal force. In the lunar day there is a dis- 
tinct magnetic tide having two ebbs and flows. The times 
maxima are two hours, and of minima seven and a half hours, 
after the culminations. The influence of the relative positions 
of the sun and moon on the horizontal magnetic force, though 
We, at the beginning of this war, been in possession of a topo- 
ya survey of the country, like the trigonometrical survey 
of France, or the ordnanee survey of Great Britain, it 1s by no 
means improbable that there might have been already saved, in 
the increased celerity and certainty of our operations, a m 
larger amount to the treasury of the country, than the whole 
survey itself could have cost. This is one of those lessons 
which governments only learn from experience. Let us hope 
that our present costly experience may not have been thrown 
away. 
