The first part of the paper deals with the action of light in deteriorating the strength of doped and undoped linen aeroplane fabric. Sunlight is shown to be the only serious agent. A curve is giver showing the change in rate of loss of strength throughout the year, the effect being very great in summer and practically negligible in winter. A comparison of the weathering properties of German and British material is given. Experiments are described which indicate the loss of strength is due to the action of ozone formed by ultra-violet liglit, the :distribution of the destructive power in the spectrum corresponding very welLwith a photo-electric theory of the formation of ozone proposed F. A. Lindemann. The efhciency of dyes and pigments as protective agents is shown to be excellent, but the latter are to be preferred on account of their effect in improving the tautness given by the dope.

The second part of the paper deals with tautness and its measurement, the more important methods are discussed, and a standard one proposed l)y which The tautness can be expressed directly as a tension. A description of the standard frames and the system of rneasurement of tautness at high humidities at presc'nt used at the R.A.E. is given. A theory to account for the tautening effect of dope is suggested.