THE annexed illustration represents a perspective view of a sud motor constructed by the writer and put in operation last summer. This mechanical device for utilizing the sun's radiant heat is the result of experiments conducted during a series of twenty years; a successicn of experimental machines of similar general design, but varying is detail, having been built during that period. The leading feature of the siin motor is that of concentrating the radiaDt heat by means of a rectangular trough having a curved bottom lined on the is- side with polished plates so arranged thattheyreflect the sun's rays toward a cylindrical heater placed longitudinally above the trough. This heater. it is scarcely necessary to state, contains the acting medium, steam or air, employed to transfer the solar energy to the motor; the trflsfer being effected by meaDS of cylinders provided with pistons and valves resembling those of motive engines of the ordinary tvoe. that solar energy cannot be rendered avaiJable for producing' motive power, in consequence of the feebleness of solar radiatioD. The great cost of large reflectors, and the difficulty of producing accurate curvature on a large scale, besides the great amount of labor called for in preventing the polished surface from becoming tarDished, are objections which have been supposed to render direct solar energy practically useless for producing mechanical power. The device under consideration overcomes the stated objections by very simple means, as will be seen by the follow- ig description. The 'bottom of the rectangular trough coDsists of straight wooden staves, supported by iron ribs of parabolic curvature secured to the of the trough. On these staves the reflecting plates, consisting of flat window glass silvered od the under side, are fastened. It will be readily understood that the method thus adopted for concentrating the radiant heat does not call for a structure of great accuracy, provided the wooden staves are secured to the iron ribs in such a position that the silvered plates attached to the same reflect the solar rays toward the heater. Fig. 2 represents a transverse section of the latter, part of the bottom of the trough and sections of the reflecting plates; the'direct and reflected solar rays being indicated by vertical aDd diagonal lines. Referring to the illustration, it will be seen that the trough, 11 feet long and 16 feet broad, including a parallel opening in the bottom, 12 in. wide, is sustained-by a light truss attached to each eDd; the heater beiDg supported by vertical plates secured to the truss. The heater is 6M inches in diameter, 11 feet long, exposing 180x9 8=1,274 superficial inches to the action of the reflected solar rays. The reflecting plates, each 3 inches wide and 26 inches loDg, iuter- cept a sunbeam of 130X ISO = 23,400 square in. section. The trough is supported by a central pivot round which it revolves. The change of inclination is effected by means of a horizontal axie—concealed by the trougb—the entire mass beiDg so accurately balanced that a pull of 5 pounds applied at the extremity enables a person to change the inclination or cause the whole to revolve. A single revolution of the motive engine devel o p s more power than needed to turn the trough and regulate its inclination so as to face the sun during a day's operation. The motor shown ' by the illustration is a steam engine, the workiDg cylinder beiDg 6 inches ill diameter with Sin. stroke. The piston rod, passing through the bottom of the cylinder, operates a force pump of 5 in. diameter. By means of an ordinary cross- head secured to the piston rod below the steam cylinder, and by ordinary connecting rods, motion is imparted to a crank shaft and fly wheel, applied at the top of I he engi ne frame; the object of this arrangement beiDg that of showing the capability of the engiDe to work either pumps or mills. It should be Doticed that the flexible steam pipe employed to convey the steam to tht' engine, as well as the steam chamber attached to the upper end of the heater, have been excluded iD the illustration. The average speed of the engine during the trials last summer was 120 turns per minute, the absolute pressure on the working piston beiDg 35 lb. per square inch. The steam was worked expansively iD the ratio of 1 to 3, with a nearly perfect vacuum kept up in the condenser in closed in the pedestal which lupporti the engine frame, In view of the foregoiDg, experts Deed not be told that the sun motor can be carried out od a sufficient scale to benefit very materially the sunburnt regions of our planet. With reference to solar temperature, the power developed by the sun motor establishes relations between diffusion and energy of solar radiatioD which show that NewtoD's estimate of solar temperature must be accepted. The following demonstration, based on the foregoiDg particulars, will be readily comprehended. The area of a sphere whose radius is equal to the earth's mean distance from the sun being to the area of the latter as 214'5":1, while the re ERICSSON'S SUN MOTOR, ERECTED AT NEW YORK, 1883. flector of the solar motor intercepts a sunbeam of 23,400 square inches section, it follows that the reflector will receive the heat developed by gg'g.g? = O'50S square inch of the solar surface. Hence, as the heater of the motor contains 1,274 square inches, we establish the fact that the reflected solar rays acting on the same are diffused in the ratio of 1,274:0 '508 = 2,507:1. Practice has now shown that, not withstanding this extreme diffusion, the radiaDt energy transmitted to the reflector, by the sun, is capable of imparting a temperature to the heater of 520° F. above that of the atmosphere. The practical demonstration thus furnished by the sun motor enables us to determine with sufficient exactness the minimum temperature of the solar surface. It also enables us to prove that the calculations made by certain French scientists, indicating that solar temperature does not exceed the temperatures produced in the lahoratory, are wholly erroneous. Had Pouillet known that solar radiation, after suffering a two thousand jive hundred and sevenfold diffusion, retains a radiant energy of 520° F., he would Dot have asserted that the temperature of the solar surface is 1,760° C. Accepting NewtoD's law that “the temperature is as the density of the rays,” the temperature imparted to the heater of the sun motor proves that the temperature of the solar surface cannot be less thaD 520° x 2,507 =1,303,640° F. Let us bear in miDd that, while attempts have been made to establish a much lower temperature than NewtoD's estimate, do demonstration whatever has yet been produced tending to prove that the said law is unsound. On the contrary, the most careful investigations show that the temperature produced byradiant heat emanating from iDcaDdescent spherical bodies diminishes inversely as the diffusion of Ihe heat rays. Again, the writer has proved by bis vacuum actinometer, inclosed in a vessel maintained at a constant temperature during the observations, that for equal zenith distance the intensity of solar radiatioD at midsummer is 5'48° F. less thaD duriDg the wiDter solstice. This diminution of the sun's radiant heat iD aphelion, it will be found, corresponds within 0'40° of the temperature which Newton's law demands. It is pi'oposed to discuss this brauch of the subject more fully on a future occasion. The op ration of ihe sun motor, it will be well to add, furnishes another proof in support of NewtoD's assumption that the energy increases as the density of the rays. The foregoing explanation concerning the reflection of the rays— see Fig. 2—shows that no augmentation of temperature takes place duri ng their transmission from the reflector to the heater. Yet we find that an increase of the number of reflecting plates increases proportionably the power of the motor. Considering that the parallelism of the rays absolutely prevents augmentation of temperature duriDg the transmission, it will be asked: What causes the observed increase of mechanical power? Obviously, the eDergy produced by the increased derrnty of the ravs acting on the heater. The truth of the Newtonian doctrine, that the energy increases as the density of the rays, has thus been verified by a practical test which caDnot be questioned. It is scarcely necessary to observe that our computation of tem perature — 1,303,640° Fah.—does not show maximum solar intensity, the following poiDts, besides atmospheric absorption, Dot having beeD considered: (1) The diminution of eDergy attending the passage of the heat rays through the substaDce of the reflect iDg plates; (2) the diminution consequent od the great amount of lieat radiated by the blackened surface of the heater; (3) the diminution of temperature in the heater caused by convection. J. Ericsson.