The pen arm is actuated by a modified alarm clock mechanism controlled by an electro-magnet, the advantage being that the clock-spring and not the magnet does the work of raising the arm. Removed from its case and with the hands and balance-wheel detached, the mechanism is mounted with the hand- spindle pointing horizontally forwards, a t a height of 46 mm above the bottom of the chart and 186 mm from the axis of the drum. The pen arm is mounted on the minute-hand spindle.

A small steel weight, consisting of two Meccano fishplates, is bolted to the pallets of the escapement, and is adjusted so that gravity just pulls the pallets to one side. The electro-magnet, salvaged from a bell indicator, attracts the fishplates, and is wired in series with the anemometer contact, power being obtained from the mains through a transformer. Thus, at each contact, the escapement advances by one tooth and the pen is raised about 0 - 2 mm.

The pen is reset to zero by a clockwork motor which rotates the cam (top centre of photograph) thus depressing the arm. Behind the motor on the cam- shaft is an escapement device, details of which are shown in the diagram of which the lower part is an elevation, with the cam-shaft a t 90' to the plane of the paper, and the upper part a plan drawing. The link carrying the pallets A and B slides horizontally, being driven by the synchronous electric motor through a crank which makes two revolutions per hour. The pallets engage alternately the two projecting studs on the cam-shaft so that in the course of half-an-hour the cam-shaft advances through angles of 20' and 340° respectively. The 340" revolution resets the pen.

This instrument has been working continuously for some time and, within its limitations, has produced excellent results.

CIRRUS WITH REMARKABLE RIBBON STRUCTURE (See opposite page) P. M . Saunders, Imperial College, London

The in- dividual ribbons of the very tenuous ice cloud were several kilometres in length but only a few hundred metres wide. They were probably fallstreaks from the altocumulus seen in the lower part of the photograph. The altocumulus which with the cirrus travelled from left to right in the photograph had the common patterned structure best described as ' fish scales ' ; the mother cloud of each ribbon was by inference an individual altocumulus scale.

The photograph was taken at Harrow on 29 December 1956.

Cirrus with remarkable ribbon structure : Harrow (see opposite page) Photograph by P. AI. Saunders

' Rime.' Thin sheets of ice formed on the leading edges of leaves and stems of an orna- mental shrub (Aznva Microphylla), growing in a windy situation a t the corner of a building. Large leaves about 4 in. long. Photographed at Rothamsted Experimental Station, Harpenden, Herts a t 10 a.m. on 31 January 1958. .4ir temperature a t 9 a.m. 30.r°F with fog, night minimum zS.9OF.

Photogrnph by F . D. Cowlaiid