2 ;6 GEODÆSIA Improved.

6. The magnetic Amplitude, is an Arch of the Horizon, in-tercepted between the East and West Points of the Compass, andthe apparent rising or setting of tne Sun from the above-mentionedPoints, which is found by observing the Sun either at rising orsetting, with an Amplitude Compass.

7. Amplitude is also an Arch of the Horizon, interceptedbetween the Sun and the East and West Points of the Horizon,and may be found by this Analogy :

As Sine Com. Lat. is to Radius, so is Sine of the Sun’s Decli-nation, to Sine of the Amplitude.

8. If the Magnetic, and the true Amplitude are both alike,(which seldom happens) then there is no Variation ; but if theydiffer, their Difference rightly counted, is the Variation.

Nate I st. The Amplitudes, both North, or both South, theirDifference is the Variation ; but one North and the other South,their Sum is the Variation.

2. Both Amplitudes easterly or westerly, if the true Ampli-tude be to the Right-hand of the Magnetic, the Variation isEast; but when it is to the Left-hand, then the Variation isWest. An Example or two will make it easy.

Example I.

Suppose the true Amplitude at Sun rising be East, 28 northerly,and magnetic Amplitude be East, 9 Degrees northerly, their Dif-ference is 19, the Variation. Now, as the magnetic Amplitudeis more remote from the North than the true, therefore the Varia-tion is westerly. If this had been at Sun setting, the Variationwould have been easterly.

Example II.

At Sun-setting let the true Amplitude be West, 21 northerly,and the magnetic 14 West southerly, their Sum is 35, the Varia-tion easterly.

It