RUDIMENTARY NAVIGATION. ROBERTSON, head-master of the Royal Academy at Portsmouth, in the last century, aptly described Navigation as "the art of conducting a ship from one port to another; it is our purpose here to show how this is accomplished, and how a ship's position is determined day by day :—1, by keeping the log and working it off; and 2, by finding the latitude and longitude, from observations of the sun, taken by means of the quadrant or sextant, together with the use of the chro nometer. PART I. HOW TO KEEP THE LOG AND WORK IT OFF. THE ARITHMETIC OF NAVIGATION. Our first step must be to have a clear knowledge of those parts of arithmetic that constantly come into use in Navigation: they are, 1, the addition, subtraction, multiplication, and division of compound quantities; and 2, decimals. As you can make no progress unless you understand these, I will delay you a few minutes to explain the method of treating them. In the measurement of time the day is used, reckoned through 24 hours without the distinction of A.m. and P.M.; then, as in the ordinary way, every hour is divided into 60 minutes, and every minute into 60 seconds; the three denominations are marked thus (h m s), and 18h 40m 21 express 18 hours, 40 minutes, 21 seconds. In the measurement of space, often called angular measure, degrees, minutes, and seconds are used; every degree is divided into 60 parts, called minutes; and minute into 60 parts, called seconds; these three every denominations are marked thus (°'"), and 8° 43' 17" express 8 degrees, 43 minutes, 17 seconds. Note.—The symbols of time and space should never be used the one for the other, since 1 hour is the equivalent of 15 degrees. In the following examples of addition and subtraction of degrees, &c., and of hours, &c., it will be found that the principle is exactly similar to compound addition and subtraction of pounds, shillings, and pence: In the first example the sum of 58" and 51" equals 109", which, on dividing by 60, make 1' 49"; therefore, writing down 49", we carry on I' to be added to the other minutes, which now amount to 102'; these, being also divided by 60, give 1° 42'; therefore, writing down 42', we carry on to to be added to the other degrees, which now amount to 77°. The other examples are similarly treated. RUDIMENTARY NAVIGATION. ROBERTSON, head-master of the Royal Academy at Portsmouth, in the last century, aptly described Navigation as "the art of conducting a ship from one port to another;" it is our purpose here to show how this is accomplished, and how a ship's position is determined day by day-1, by keeping the log and working it off; and 2, by finding the latitude and longitude, from observations of the sun, taken by means of the quadrant or sextant, together with the use of the chronometer. PART I. HOW TO KEEP THE LOG AND WORK IT OFF. THE ARITHMETIC OF NAVIGATION. OUR first step must be to have a clear knowledge of those parts of arithmetic that constantly come into use in Navigation: they are, 1, the addition, subtraction, multiplication, and division of compound quantities; and 2, decimals. As you can make no progress unless you understand these, I will delay you a few minutes to explain the method of treating them. In the measurement of time the day is used, reckoned through 24 hours without the distinction of A.M, and n B P.M.; then, as in the ordinary way, every hour is divided into 60 minutes, and every minute into 60 seconds; the three denominations are marked thus (hms), and 18h 40m 21s express 18 hours, 40 minutes, 21 seconds. In the measurement of space, often called angular measure, degrees, minutes, and seconds are used; every degree is divided into 60 parts, called minutes; and every minute into 60 parts, called seconds; these three denominations are marked thus ("), and 8° 43′ 17′′ express 8 degrees, 43 minutes, 17 seconds. Note. The symbols of time and space should never be used the one for the other, since 1 hour is the equivalent of 15 degrees. In the following examples of addition and subtraction of degrees, &c., and of hours, &c., it will be found that the principle is exactly similar to compound addition and subtraction of pounds, shillings, and pence: In the first example the sum of 58′′ and 51" equals 109", which, on dividing by 60, make 1' 49"; therefore, writing down 49", we carry on l' to be added to the other minutes, which now amount to 102'; these, being also divided by 60, give 1° 42′; therefore, writing down 42', we carry on 1° to be added to the other degrees, which now amount to 77°. The other examples are similarly treated. We now proceed to subtraction. Ex. From 44° 13′ 18′′ Ans. 26 48 46 Ex. From 11h. 48m. 52s. Ans. 2 4 1 As before observed, this operation is similar in principle to common compound subtraction, commencing on the right hand, and when required, adding the components (60" or 60') of one of the next denomination to the number subtracted from; thus, when we have to subtract 32" from 18", we must add 60" to the 18"; then 32" subtracted from 78" leave 46"; having thus borrowed 1', we have to add it to the 24', making now 25' to be subtracted from 13', when we again proceed in a similar manner. It will frequently be necessary to subtract an upper line of figures from a lower, and often similarly to the following examples :— In the first example we had to borrow consecutively 60" and 60'. We shall also have at times to multiply degrees, &c., and hours, &c., thus Here 46" multiplied by 4 equal 184", which, when divided by 60, equal 3' 4"; we put down 4", and carry 3' to the product of 4 times 35, which gives us 143′; |