138 U. S. COAST AND GEODETIC SURVEY 
record is limited by the width of the paper which extends 3 inches on 
either side of the medial line, but for mechanical reasons it is desirable 
in general to keep the record within a band 2% inches on either 
side of the medial line. The following table suggests suitable scale, 
dial, and gear combinations for different tidal ranges and different 
current velocities. The tabular marigram scales are applicable only 
when the foot or knot has been used as the unit for machine settings. 
The marigram amplitude limits given in the last column are expressed 
in the same unit that is used in setting the machine regardless of what 
unit that may have been. 
Working scale, height dial, marigram gear, and scale 
é lee Sikes Marigram scale Mari- 
- urren orking ‘ Bigs |e _| gram 
ycel renee velocity scale Boe gram ampli- | 
3 limits factor gear Settings | Settings tude | 
F in feet | inknots | limit | 
: Knols 
Feet Knots | Ratio per inch Units 
' 0.0- 2.5 0.0- 1.0 10 4 eA 1: 6 0. 50 1.5 
{ 2.6- 3.5 l- 1.5 10 4 3:2 NS" f} 0. 67 2.0 
3. 6- 4.0 1.6- 2.0 10 4 Tea 1:12 1.00 3.0 
4 1- 6.0 2.1- 3.0 4 10 2:1 11S 1.25 3. 
6. 1- 8.0 3. 1- 4.0 4 10 3:2 1:20 1. 67 3.0 | 
8. 1-10.0 4.1- 5.0 4 10 Pil 1:30 2. 50 7.5 
10. 1-12. 5 5. 1- 6.0 2 20 221 1:30 2. 50 7.6 
12. 6-16. 5 6.1- 8.0 2 20 3:2 1:40 3. 33 10.0 
16. 6-20. 0 8. 1-10.0 2 | 20 Mea 1:60 5. 00 15.0 
20. 1-25. 0 10. 1-12. 5 1 40 2:1 1:60 5. 00 15.0 
}) 25. 1-32. 5 12. 6-16. 0 1 40 3:2 1:80 6. 67 20.0 
i) 32. 6- 16. 1- 1 40 1:1 1:120 10. 00 30.0 
L 
When height dial readings are not required, and amplitude settings are in feet, a convenient graph scale 
‘of 1:10 can be obtained by using any one of the following combinations; seale factor 12 with gear ratio 1:1, 
-seale factor 8 with gear ratio 3:2, or scale factor 6 with gear ratio 2:1. 
398. When the tide-predicting machine is used for the prediction 
of the tide-producing force, the graph scale to be adopted will depend 
upon the unit in which the force is to be expressed. Assume that the 
sum of all terms in the vertical component of the force (par. 79) is 
desired. Referring to paragraph 43, it will be noted that the extreme 
value of this component due to the combined action of moon and sun 
is approximately 0.2 <10~° with the unit of force taken as g, the mean 
acceleration of gravity. In this case a convenient scale relation which 
will bring the graph within the desired limits on the paper is obtained 
by adopting a working scale factor of 610’ with the marigram gear 
ratio of 2:1. With this combination 0.1 foot of graph ordinate will 
represent 1077 g units of force. In practice the scale factor would be 
combined with the general coefficient common to all terms in the 
formulas. 
399. Pens.—The curve-line pen (13, fig. 23) and the datum-line pen 
(14) are each of the ordinary fountain type. Each is fitted with a 
metal lock joint, so that it may be quickly removed and replaced in 
the same position, and is pressed against the paper by a light coil 
spring when in use. The curve-line pen is mounted in a swivel arm 
on a light carriage which slides vertically along two rods. The datum- 
line pen is mounted in a swivel arm that may be adjusted so that the 
mean sea-level line will be traced midway between the upper and lower 
edges of the paper. 
