페이지 이미지
PDF
ePub
[blocks in formation]

but the decimal part from Table 42 remains the same.

MULTIPLICATION

To multiply one number by another, select the logarithms from Table 42, and when properly indexed their sum will be the logarithm of the product. This logarithm, when found in Table 42, will be at the intersection of some figures in the first column and under one of the numbers at the top of the page. These four figures pointed off from the right, so there will be one more than the index, will be the product wanted.

[merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small]

Example: Multiply 92 by 8 and the product by 6.

[blocks in formation]

DIVISION

To divide one number by another, select the logarithms from Table 42 and when properly indexed, subtract the logarithm of the divisor from that of the dividend, adding 10 to the index of the logarithm of the dividend if necessary to make subtraction possible, and the re nainder will be the logarithm of the quotient. Example: Divide 20.01 by 8.7.

index 1.30125

8.7) 2 0.0,1(2.3 Proof - 0.93952

174 Ans. 2.300

= 0.36173

261

20.01

[ocr errors]

8.7

261

Example: Divide 7 by 8. 7.

index + 10 = 10.84510 8.

0.90309 Ans. .875

= 9.94201

8)7. (.875 Proof

64

60 56

40
40

LOGARITHMS FROM TABLE 44 For the solution of many problems in navigation the use of logarithms from Table 44 is necessary and should be thoroughly understood. In this work they are used in the sailings.

The degrees begin at the top of the page and run to the right to 45°. From that they run on the bottom to the left to 90° and then on the bottom to the right to 134o. From 135° to 180° they run to the left at the top of the pages. The degree of the angle being located at one side or the other of the page, the miles must be taken from the mile column on the same side. If the degrees are found at the top, the various names must also be read from there and when the degrees are at the bottom of the page the names over the columns must be read from the bottom.

If the logarithm wanted is to be corrected for seconds, the logarithm for the lesser mile is taken and then find the seconds in the left-hand mile column. The correction will be found opposite in the column of differences next to that from which the loragithm is taken and is added or subtracted, according to the next logarithm being more or less.

Up to and including 4° and other angles on the same pages, the correction columns contain the difference for i'. When the angle falls on those pages, the correction for seconds is found by taking the number directly opposite, dividing it by 60 and then multiply by the number of seconds or reduce the seconds to a decimal part of a mile and by it multiply the number from the correction column. The logarithm is taken for the nearest mile when a correction column is not given.

1. Example: Find the logarithm sine for 35° 25' 28”.

[blocks in formation]

COS.

2. Example: Find the logarithm cosine for 125° 54' 42". 125° 54'

9.76817 42"

+ 125° 54' 42"

9.76829

cor.

12

= COS.

COS.

3. Example: Find the logarithm cosine for 144° 0' 50". 144° oo'

9.90796

+ 8 144° 00'50"

9.90804

50"

cor.

=COS.

4. Example: Find the logarithm tangent for 54° 20' 54". 54°20'

tang. .14406 54"

cor. + 24 54° 20' 54"

tang. .14430

5. Example: Find the logarithm tangent for 35° 50' 38". 35° 50'

tang.

9.85860

+ 17 35° 50' 38"

tang. 9.85877

38"

cor.

6. Example: Find the logarithm cosine for 54° 27' 40". 54° 27'

9.76448

COS.

[blocks in formation]

7. Example: Find the logarithm cosine 35° 10' 46".

9.91248

7 35° 10' 46"

9.91241

35° 10'

cos.

46"

cor.

= COS.

8. Example: Find the logarithm cosine for 86° 28' 42". 86° 28'

COS. 8.78979

Dif. 1 204 cor. for 42" = 204 X .7

143 42" = X .7 86° 28' 42"

=cos. 8.78836 143 = 142.8

[blocks in formation]

DEFINITIONS Compass course is that course steered by the ship's compass. It includes variation and deviation, heeling error, current and leeway, if any.

Magnetic course is the course steered by the ship's compass when free from deviation and other influences.

True course is the course obtained by applying the variation, deviation and leeway, if any.

Leeway is the angle between the ship's keel and the direction being made through the water.

Variation is the quantity the north point of the compass is east or west of the true north. It is the earth's effect on the compass and is not the same in all localities. It is always increasing or decreasing.

Deviation is the quantity the north point of the compass is east or west of the magnetic north. It is the result of the ship's effect on the compass.

It is different for every point of the compass and, therefore, changes every time the course is altered.

Compass error is the quantity the north point of the compass is east or west of the true north.

If the compass is free of deviation, it will be the same as the variation.

Distance is the number of miles the ship makes on any one course or the number of miles between any two points on the earth's surface.

Distance made good is the number of miles a ship is from the point she started regardless of the distance she has sailed.

Departure course is the bearing reversed of the fixed object on shore from which a ship takes her departure upon leaving port. The distance is also noted.

Wind is air in motion and its direction is named after the point from which it blows.

Current is water having progressive motion. Its set is named after the point from which it flows and its drift is expressed in miles either hourly or daily.

Difference of latitude is the quantity taken for each corrected course and distance from the latitude column in Tables I or 2, taking care to use the top or bottom according to

« 이전계속 »