50 MANUAL OF CURRENT OBSERVATIONS 
Taste 4.—Rating table for Ekman current meter (applicable to meter No. 110) for 
1-minute time interval 
Screw | Screw Screw | Screw Serew | Screw Serew | Screw Serew | Screw 
Rev. |No, 219|No. 220|| ReV- |No. 219|No. 220|| Rev- |No. 219|No. 220]] ReV- |No. 219|No. 220|| ReY- |No. 219/No. 220 
Knots | Knots Knots | Knots Knots | Knots Knots | Knots Knots | Knots 
Mises 0. 13] 0. 14]} 40___] 0. 46) 0. 48|| 70___| ©. 79} 0. 83}} 100__| 1. 12) 1. 18)} 1380__] 1. 45) 1. 53 
20s 2e . 24) . 25)| 50___| . 57] .60|/ 80___| . 90) . 95)|-110__} 1. 23) 1. 30)| 140__| 1. 56) 1. 64 
350 . 35} . 37|| 60___| . 68 ae 90___| 1. 01) 1. 06)| 120__| 1. 34) 1. 41)} 150__} 1. 67) 1. 76 
vided each dial reading is always subtracted from the preceding reading, the latter 
being increased by 4,000 when less than the following reading. However, if the obser- 
vational interval is long and the current velocity large, the total number of revolutions 
of the meter screw during a single observational interval may exceed 4,000, a fact 
which might not be immediately obvious in the record itself except by inference based 
upon a general knowledge of the approximate current velocity to be expected at the 
time. For example, if the observational interval used is 30 minutes, a count of 4,000 
by meter will represent an average of 133 revolutions per minute which in the rating 
table indicates a velocity of about 1% knots. If it is known that the current velocity 
is approximately 1% knots and the count during the 30-minute interval indicates a 
much smaller velocity, it may be assumed that 4,000 revolutions should be added to 
the count. A longer observational interval will increase the possibility of exceeding 
this limit, while a shorter interval will make less likely the need for taking this matter 
into account. 
Tas.E 5.—Rating table for Roberts radio current meter 
Contact interval Velocity Contact interval | Velocity || Contact interval] Velocity || Contact interval] Velocity 
Seconds Knots Seconds Knots Seconds Knots Seconds Knots 
36 —20 2 2. 63-2. 48 1.6 1. 32-1. 29 3. 1 0. 88-0. 87 4.6 
19° —12: 9 3 2. 47-2. 33 il, @ 1. 28-1. 24 3. 2 . 86— . 85 4.7 
12. 8 9.8 4 2. 32-2. 20 1.8 23-12 3.3 84— . 83 4.8 
9 7— 7.9 5 2. 192.09 1.9 1. 20-1. 17 3. 4 . 82 4.9 
2. 08-1. 99 2.0 1. 16-1. 14 3.5 8l- . 78 5.0 
ee see “8 || 1, 98-1. 89 Ri |i T, NM, Ti 3.6 i= | BD 
5 6 5.0 “8 1. 88-1. 81 2.2 1. 10-1. 08 3.7 75— . 73 5. 4 
NOL AA 9 1. 80-1. 73 | 2.3 1. 07-1. 05 3. 8 C= - Ul 5. 6 
43-3 95 L 0 1. 72-1. 66 | 2.4 1. 04-1. 02 3.9 70— . 68 5.8 
: ; ? 1. 65-1. 59 2.5 1. 01-1. 00 4.0 67— . 66 6.0 
| 3. 94-8. 59 iL 1. 58-1. 53 2. 6 0. 99-0. 97 4.1 65— . 64 6. 2 
3. 58-3. 30 1.2 1. 52-1. 47 2. - = 5 0) 4.2 || 63-— . 62 6. 4 
| 3. 29-3. 05 1.3 1. 46-1. 42 2.8 . 94— . 93 4.3 || 61-— . 60 6. 6 
| 93. 04-2. 83 1. 4 1. 41-1. 37 2.9 ~ 92> . Oil 4.4 . 59 6. 8 
| 2. 82-2. 64 1.5 1. 36-1. 33 3.0 . 90- . 89 4.5 . 58— . 57 7.0 
For reference to this table see par. 87. 
Record of Current Direction 
129. By direction of current is meant the direction toward which the water moves. 
It is convenient to express this in degrees of azimuth as reckoned clockwise from the 
north. For short series of observations it is the general practice to reduce the direc- 
tion of each individual observation to the true azimuth, this reduction being made on 
