EPICYCLOID. A geometrical curve generated by making a circle roll upon the circumference of another circle; it is found useful in determining the figure of the teeth of wheel-work, and other purposes in mechanics. If the generating circle proceeds along the convexity of the periphery, it is called an upper or exterior epicycloid; if along the concavity, a lower or interior epicycloid.
EPOCH. The time to which certain given numbers or quantities apply.
EPROUVETTE. A small piece of ordnance specially fitted for testing the projectile force of samples of gunpowder.
EQUATED ANOMALY. This is also called the true anomaly, and is the distance of the sun from the apogee, or a planet from its aphelion, seen from the sun.
EQUATION, Annual. See [Annual Equation].
EQUATION OF EQUINOXES. The difference between the mean and apparent places of the equinox.
EQUATION OF THE CENTRE. The difference between the true and mean anomalies of a planet.
EQUATION OF TIME. The difference between mean and apparent time, or the acceleration or retardation of the sun's return to the meridian.
EQUATOR. Called also the equinoctial line, or simply the line, being an imaginary circle round the earth, dividing the globe into two equal parts, and equally distant from both poles. Extended to the heavens, it forms a circle called the celestial equator, which in like manner divides the heavens into two equal parts, the northern and southern hemispheres.
EQUATORIAL CURRENT. The set, chiefly westerly, so frequently met with near the equator, especially in the Atlantic Oceans.