Construction

Hollow tube with an eye piece at one end and a short sighting tube fitted at the other.

Eye piece consists of 2 or 3 telescopic hollow tubes and a sighting tube is a small detachable tube fitted with a horizontal wire at the centre. A mirror behind the horizontal wire covering only half of the tube so fitted that it makes 45°.

A spirit level is fitted to the main tube , which can be rotated by one screw. Wheel is for quicker movement and screw is for final adjustments.

An index arm is also attached to the spirit level. As the spirit level rotated, index arm moves on a graduated semi-circular arc. As the angles of elevation and depression are needed, graduation in degrees up to 900.

On either side of the zero mark at middle.Each division gives the reading of 10’. It is based on trigonometrically principles.

It is commonly used instruments. It has eye piece (hollow telescopic piece), magnifier glass, protector, wire, screw.

Methods of Use

Observer should look through the eyepiece and coincides with top of tree and wire, the spirit should be balanced . It should be balanced by tuning with screw as well as moving to and back ward.

Advantages

• It gives accurate angle of elevation anddepression.
• Readings can be taken after sighting the tree without disturbing the index arm.
• The instrument is small and light and can be used even in hills without difficulty.

Disadvantages

• Shaking of the hand makes the sighting of the top and bottom of the tree a little difficult and time-consuming.
• Sprit level needs adjustment , which is quite tiresome