408 
will still, it is to be hoped, cover with noble 
forests the highest and most inaccessible slopes 
of the Rocky Mountains, recalling to men, 
as long as the study of trees occupies their 
thoughts, the memory of a pure, upright, and 
laborious life. 
THE MAXIMA AND MINIMA TIDE- 
PREDICTING MACHINE.! 
Tuts machine has been invented by Mr. 
Ferrel, and constructed by Fauth & Co. of 
Washington, for the use of the Coast and geo- 
detic survey. Its object is to determine me- 
chanically the times and heights of high and low 
waters for the numerous tide-stations around 
our coast, for which tide-tables are annually 
published. The numerical data for these have 
been heretofore obtained by computation ; but, 
on account of the great complexity of the tidal 
theory and formulae, this involves a great 
amount of labor to obtain even approximate , 
results, and more accurate ones have to be 
dispensed with, unless they can be obtained 
in some way mechanically, with much greater 
facility than by computation. 
The first tide-predictor was invented by Sir 
William Thomson, about eight years ago. 
This was constructed so as to take into account 
about ten only of the principal tide-components ; 
all, however, which are of much practical im- 
portance. ‘This machine has not been used in 
the regular prediction of tides, and is said to 
be now on exhibition at the South Kensington 
museum. 
Subsequently Mr. Roberts, of the Nautical 
almanac, London, had another constructed 
upon somewhat the same plan, but larger, tak- 
ing into account twice as many of the compo- 
nents, and having some improvements on the 
plan introduced. A description of this ma- 
chine was given in The engineer of Oct. 19, 
1879. It is now being successfully used in 
the prediction of the tides of India. 
Both of these machines have been constructed 
so as to be run by clock-work, and to give the 
results in the form of a tide-curve for one year 
ona roll of paper, from which the times and 
heights of high and low waters are afterwards 
read off and recorded. 
In the maxima and minima predictor, only 
the maxima and minima of the heights of the 
tide above mean low water or any other as- 
sumed plane of reference, and the times of their 
occurrence, are indicated; as these alone are 
1 This article, written by Mr. WILLIAM FERREL, is published 
by permission of the superintendent of the Coast and geodetic 
8urveye 
SCIENCE. 
>. eye n 
[Vox IIL, No. 61. 
required for the tide-tables annually published. __ 
For this purpose a transformation of the tidal 
harmonic function was necessary, so that it 
would give heights and times of the maxima 
and minima; and, as any such transformation 
usually renders the resulting expression much 
more complex than the original one, the whole 
theory and construction of this machine is 
much more complex than in the case in which 
the machine is required to give the height of 
the tide at regular stated intervals of time, or 
a graphic representation of the whole function. 
In this machine both the clock-work and roll 
of paper are dispensed with, and the machine 
is run by means of a small crank at the side, 
with the left hand; and the times and heights 
of high and low waters are read off from the 
face of the instrument, and recorded as you go, 
with the right hand, upon blank forms ready 
for the printer. The great advantage which is 
claimed for this form of the machine is that it 
gives only what is required, and this in such a 
way that the results can be recorded at once, 
and the trouble of handling long rolls of paper, 
and estimating the times of maxima and mini- 
ma, and reading off the corresponding height 
of the tide, is saved. Although the machine is 
more complex, this makes no difference in the 
. facility with which the results are obtained. 
The crank is turned until an index on the cen- 
tral dial of the face, called the lunar index, 
pointing between eight and nine on the accom- - 
panying perspective representation of the face 
of the instrument, comes in conjunction with 
the upper end of an oscillating needle, the upper 
end pointing between twelve and one, as rep- 
resented, when the time of high water is pointed 
out by another index on the same dial, called 
the solar index, pointing, as represented, to the 
figure twelve, at noon and midnight, and the 
height is indicated by an index on a vertical 
scale on the left side of the face of the instru- 
ment. You then turn until the lunar index 
comes in conjunction with the lower end of 
the needle, when the solar index pointsout the 
time of low water, and the index at the side, its — 
height. Turning until the lunar index comes 
in conjunction again with the upper end of the — 
needle, you read off, as before, the times and 
heights of the next high water ; and so on from 
high to low and from low to high water through 
the year, recording the results as you go. 
Where, however, there are large diurnal com- 
ponents, it is necessary to run through twice, — 
first for the times, and then, after a little change 
in the setting, for the heights. ‘The machine, — 
therefore, is especially convenient for most of 
the tides having a large range upon our At- © 
