48° Report of the U. 8. Coast Survey for 1883. 
direction, for predicting the times and heights of high and low 
water for any given station. 
n Thomson’s machine and its successor—now in successful 
operation for predicting tides on the coast of India the machine 
traces the tidal curve on a long strip of paper. From this 
strip of paper the times and heights are to be read off and tabu- 
lated in form for printing and publication. This machine, 
therefore, makes known the height of the tide for every instant. 
In the machine now first constructed and described by Prof. 
Ferrel we obtain not a curve giving the height of the tide at 
all times; but for the particular times we desire, viz: at hig 
water and at low water we obtain the times and heights. 
The machine, therefore, which gives these results is for distine- 
tion called a “ Maxima and minima tide-predicting machine.” 
It is of interest to note that the theory upon which the ma-— 
chine solves the tidal problem is more accurate than the usual 
mode of computing. In the words of the author: 
“The formule used in the machine are those best adapted to 
obtain the results accurately by computation. This, however, 
involves so great an amount of labor that it has been neces- 
sary heretofore to use more simple formule, requiring much 
less labor in computation, but which give often only very 
rough approximations to the true results. These can now be 
pretty accurately obtained with scarcely any labor.” 
Against this greater theoretical accuracy must be set off the 
inaccuracies due to mechanical construction; but when this: 
has been done the machine still shows results quite equal to” 
the results obtained by direct computation even when tested 
by tides so different as those at Boston, San Diego and Tahiti. 
In capacity for doing work the author estimates the machine 
to be equivalent ‘to 30 or 40 computers, if these were to take 
account of everything which the machine does.” 
According to this estimate we may say that if one computer 
could by the usual mode of computation make the tide-tables 
for the Atlantic sea-coast of the United States in one year 
without the machine, then with the machine he would in one 
year predict the Atlantic tides for the next 30 or 40 yea 
. . 
rs. | 
When it is borne in mind that the constants for setting the 
