IN an interesting article, published in Engineering News, H. E. Willsie first reviews the various attempts— many ridiculous and some otherwise—which have been made toward the solution of the problem of the conversion of the solar heat available at the earth's surface into mechanical power. The experimental work of Mr. Willsie and J.Boyle, Jr., is then described at considerable length. The principle adopted is that of “indirect storage.” Thus the intermittent solar heat is absorbed and collected by water circulating under the glass of the “heater.” The water so heated is stored in a tank, well insulated to reduce the loss by radiation. The heated water is drawn uniformly and continuously from the storage tank and made to give up its heat by circulating it about a boiler containing SO2. The vapor generated by this heat, after operating an engine, is condensed and returned to the boiler to be used again, while the water which has given up its heat to the S02 is sent through the solar “heater” again to collect more heat from the sun's rays. It is found that on parallel 34 deg. about 2,300 British thermal units per square foot will be taken up by the water during a June day and about 1,600 British thermal units during a December day. The experiments finally led up to a 20-horse-power sulphur dioxide engine being used, which at times, with a boiler pressure of' 215 pounds per square inch developed about 15 horse-power. Two heater sections exposing an area of 1,000 square feet to the sun (each square foot absorbs 'about 377 British thermal units per hour) and a condenser were used, the water for which latter was pumped by the engine from a well. The following figures, based Upon Josse's data, are given as to cost of a solar power plant: Heater, 24-hour size, per horse-power... $100.00 Storage, 100-hour size, per horse-power. 10.00 Engine, pumps, etc., per horse-power.. 20.00 Vaporizer, per horse-power 15.00 ' Condenser, per horse-power 15.00 Liquid sulphur dioxide 1.25 Emergency steam boiler 2.75 $164.00 A comparison of estimated costs (on the basis of the experiments) per horse-power-hour with steam and solar-electric plants of 400 horse-power is given below: Cost of one electric horse-power- hour, cents. Steam. Solar. Engineer, 40 cents per hour 0.08 0.08 - Fireman, 30 cents per hour 0.06 Dynamo man, 40 cents per hour.. 0.08 OK Helper, 25 cents per hour 0.05 0.05 Superintendence 0.06 0.06 Coal 1.5 Interest, maintenance, depreciation power plant 0.046 0.184 Interest, maintenance, depreciation electric plant 0.006 0.006 Oil, waste, water (or sulphur dioxide) 0.15 0.15 Total, estimating steam plant at $40 per horse-power 2.032 cents Total, estimating solar plant at $164 per horse-power 0.610 ct. From these figures it is clear that to compete with a sun power plant (which of course would only be employed in suitable regions, such as in California, where the authors carried out their experiments) a steam plant- would have to obtain its coal for 66 cents per ton, or a producer-gas engine for about $2 per ton. COLOGNE, SEEN FROM the “PAltSEVAL.” a vIEw OF the TYROLESE MOUNTAINS FROM A DIRIGIBLE BALLOON ON BOARD THE PARSEVAL.