April 4, 1901] 



NA TURE 



549 



axis of the instrument is the steam delivery tube. When every- 

 thing was cold and the sun's rays were concentrated by the 

 reflector on the tube A B, the water boiled in forty seconds. 

 The steam rises in the glass dome, from which it finds exit 

 through the tube G L. Condensation begins so soon as the 

 steam has passed below the point B, and the water produced is 

 collected at L in a graduated tube. After the distillation has 

 been ruiming for a certain time a considerable quantity of very 

 hot water is collected at the upper part of the condenser, and it 

 slowly rises through the narrow annular space B c to replace the 

 water removed from the boiler by evaporation. The boiler is 

 thus fed with water at the boiling temperature, and when the 

 calorimeter has settled down into steady working, the whole of 

 the heat which reaches the water from the sun is used in 

 transforming water at its boiling point into steam of the same 

 temperature. It is essential that the distillation be kept running 

 continuously. If the meteorological conditions are such that 

 the boiling is interrupted, then it is of no use making observa- 

 tions at all. 



Locality. — Sohag, where the observations were made, lies on 

 the left bank of the Nile, in lat. 26° 37' N, The expedition 



Fig. t. 



arrived there on May 8, 1882, and I was able to begin work on 

 the nth. As the instrument was new in every way, the work 

 of the first few days was directed towards learning the manipu- 

 lations and finding out and rectifying defects. Improvements 

 of one kind or another were made every day up to the 15th. 

 On the l6th, 17th and i8th experiments were carried out with 

 the instrument in best working order, and under very favour- 

 able conditions. The sun's declination was 19° 22' on the 17th, 

 so that the mean meridian altitude during the three days 

 was 82° 45', corresponding to a zenith distance of 7° 15'. The 

 following table gives the sun's zenith distance, as taken from the 

 globe, at every half hour from noon to + 4 hours, apparent 

 time : — 



Hours . o 

 ©'s zenith 

 distance 7°"25 io°'5 16° 22°-5 29° 36° 42° 49" 5S°.S 



The useful time for observation is from 9 a.m. to 3 p.m. 



The principal object of the experiments was to ascertain the 

 maximum rate of distillation under the most favourable cir- 

 cumstances. This occurred during the forenoon of May 18, 

 when the meteorological conditions were as favourable as they 

 could be. The sun shone steadily in a cloudless sky, and the 

 air was motionless. The shade temperature reached 40° "5 C. 

 in the course of the day. 



NO. 1640, VOL. 63] 



Between nh. 35m. 40s. and iih. 39m. a.m., 5 cubic centi- 

 metres of water were distilled, being at the mean rate of I "501 c.c, 

 per minute at nh. 37m. 20s. a.m. As the collecting area of the 

 reflector is 904 square centimetres, this corresponds to i6"6oc.c. 

 distilled per minute per square metre. At 11.37 a.m. the sun's 

 zenith distance was 20°. Therefore we know that the sun's 

 perpendicular rays, as received at or near the sea-level, have a 

 heating eff"ect sufficient to evaporate tnore than i6'6c.c. of water 

 per square metre per minute. Correcting this value for the 

 obliquity of the sun's rays, by the method which shall be 

 indicated presently, it becomes 17 "04 c.c. per square metre per 

 minute. 



If we take the cubic centimetre of water to weigh one gramme 

 and the latent heat of steam at 100° C. to be 535 gramme- 

 degrees (grs.° C), the evaporation of 17 04 c.c. water requires 

 91 16 grs.° C. of heat ; and this is the amount of heat in ordinary 

 units per square metre per minute which can be collected from 

 the rays of the vertical sun at the sea-level, and can be there 

 utilised. Further, 91 16 grs.°C. of heat are equivalent to 3875 



Fig. 2. 



kilogramme-metres (kg.m.) of work, so that the working value 

 of the sun's rays as collected by our calorimeter is 3875 kg.m, 

 per minute, or o '87 horse-power per square metre. No allowance 

 has been made for instrumental imperfections. They certainly 

 exist, but I do not think that more than ten per cent, need be 

 allowed for them. If, however, we increase the working value 

 of the sun's rays from 087 to i"0 horse-power, the allowance is 

 certainly sufficient ; and this is probably very close to the true 

 heating value of the sun's rays as they reach the sea-level. 

 Taking the earth's mean distance from the sun's centre to be 212 

 times the radius of the sun, the radiation emitted by one square 

 metre of the sun's surface is spread over, in round numbers, 

 45,000 square metres of the earth's surface. Therefore the 

 intensity of the radiation of the sun's surface is equivalent to at 

 least 45,000 horse-power per square metre. This figure, 

 especially when used in connection with so very small a surface 

 as one square metre, conveys no definite idea to the mind. The 

 following consideration may assist in giving definition to our con- 

 ception. The specific gravity of solid iron at ordinary terrestrial 

 temperatures is about 75 ; therefore one cubic metre of it 

 weighs at the earth's surface 7500 kilogrammes. Taking the force 

 of solar gravity at the sun's surface to be twenty-eight times that 



