Mounting of Mirror
This mirror, to maintain its accuracy, not only requires careful mounting in its cell but also protection against temperature changes. Even though 12 inches thick it would bend under its own weight of 4,300 lbs. sufficiently to affect the figure and consequently it is supported in the cell by a specially counterweighted lever system so that it is equably supported at twelve points and there is no tendency to bend. A similar lever support system around the edge prevents distortion due to constraint when it is tipped from the horizontal position at different positions of the tube. Temperature changes can produce much greater distortion than flexure but Victoria has the advantage of very low diurnal range and the temperature change around the mirror is made very small by a lagging of cotton felt about 2 inches thick all round the sides of the closed section of the tube, laced on with a duck cover (compare Fig. 3 with Fig. 2) and an equal thickness below and around the edge of the mirror. By this lagging the temperature rise in the day-time is only about half a degree while the dome temperature increases five degrees, hence the figure of the mirror remains good whatever the temperature changes outside.