Sept. n, 1884] 



NATURE 



46 S 



more elevated point, the apparent colour of the fpray became 

 bluish. G. H. 



September 5 



Circular Rainbow seen from a Hill-top 

 Noticing a communication in Nature (p. 361) regarding 

 the phenomenon of a circular rainbow, I thought it worth while 

 to mention a case which lately came under my observation. 

 Standing on a point of rock just opposite the beautiful falls of 

 Montmorenci, Quebec, I was surprised to see a rainbow in the 

 form of a circle passing through my feet. The spray from the 

 falls was being blown into a deep cove in front of me, and the 

 sun was high in the heavens behind. The primary was well 

 defined and very beautiful ; the secondary was faint. I un- 

 derstand that the conditions for seeing this circular rainbow are 

 not often favourable at Montmorenci ; still it may not be amiss 

 to advise intending visitors not to stop at the bottom of the steps 

 which lead down below the falls, but to clamber over the rocks 

 as near the water as possible. W. L. Goodwin 



Montreal, August 28 



Intelligence in Frogs 



A FRIEND in Scotland has a small lake in his grounds, which 

 are surrounded by a high wall. At the bottom of the lake is a 

 sluice by whicli the water can be let off into a burn below the 

 grounds. A few weeks ago the lady of the house was walking 

 down the road outside the wall towards the burn when, to her 

 astonishment, she met a multitude of frogs making their way up 

 the road, which makes a considerable detour, to the gate leading 

 into the grounds. On inquiry she found that the lake had that 

 morning been emptied through the sluice, and it was plain that 

 these were frogs which, having been carried down with the water 

 to the burn, were now making their way back to their old home. 

 By what instinct did they know that the long road led to the 

 point from which the short one had started ? B. W. S. 



September 3 



THE TEMPERATURE OF THE SOLAR 

 SURFACE 

 'PHE power developed by the sun motor, recorded in 

 -*■ Nature, vol. xxix. p. 217, has established relations 

 between diffusion and energy of solar radiation which 

 prove that the temperature of the surface of the sun is 

 extremely high. I have, therefore, during the summer 

 solstice of 1884, carried out an experimental investigation 

 for the purpose of demonstrating the temperature of the 

 solar surface corresponding with the temperature trans- 

 mitted to the sun motor. Referring to the illustrations 

 previously published, it will be seen that the cylindrical 

 heater of the sun motor, constructed solely for the purpose 

 of generating steam or expanding air, is not well adapted 

 for an exact determination of the amount of surface ex- 

 posed to the action of the reflected solar rays. It will be 

 perceived on inspection that only part of the bottom of 

 the cylindrical heater of the motor is acted upon by the 

 reflected rays, and that their density diminishes gradually 

 towards the sides of the vessel ; also that owing to the 

 imperfections of the surface of the reflecting plates the 

 exact course of the terminal rays cannot be defined. 

 Consequently, the most important point in the investiga- 

 tion, namely, the area acted upon by the reflected radiant 

 heat, cannot be accurately determined. I have accordingly 

 constructed an instrument of large dimensions, a polygonal 

 reflector (see Fig. 1), composed of a series of inclined 

 mirrors, and provided with a central heater of conical 

 form, acted upon by the reflected radiation in such a 

 manner that each point of its surface receives an equal 

 amount of radiant heat in a given time. The said reflector 

 is contained within two regular polygonal planes twelve 

 inches apart, each having ninety-six sides, the perimeter of 

 the upper plane corresponding with a circle of eight feet 

 diameter, that of the lower plane being six feet. The cor- 

 responding sides of these planes are connected by flat 

 t?per mirrors composed of thin glass silvered on the out- 



side. When the reflector faces the sun at right angles, each 

 mirror intercepts a pencil of rays of 32'6i square inches 

 section, hence the entire reflecting surface receives the 

 radiant heat of an annular sunbeam of 32'6i X 96 = 3130 

 square inches section. It should be observed that the 

 area thus stated is 001 1 less than the total foreshortened 

 superficies of the ninety-six mirrors if sufficiently wide to 

 come in perfect contact at the vertices. Fig. 2 represents 

 a transverse section of the instrument as it appears when 

 facing the sun ; the direct and reflected rays being in- 

 dicated by dotted lines. The reflector and conical heater 

 are sustained by a flat hub and eight radial spokes bent 

 upwards towards the ends at an angle of 4.5 . The hub 

 and spokes are supported by a vertical pivot, by means of 

 which the operator is enabled to follow the diurnal motion 

 of the sun, while a horizontal axle, secured to the upper 

 end of the pivot, and held by appropriate bearings under 

 the hub, enables him to regulate the inclination to corre- 

 spond with the altitude of the luminary. The heater is 

 composed of rolled plate iron 0^017 inch thick, and pro- 

 vided with head and bottom formed of non-conducting 

 materials. By means of a screw-plug passing through the 

 bottom and entering the face of the hub the heater may 

 be applied and removed in the course of five minutes, an 

 important fact, as will be seen hereafter. It is scarcely 

 necessary to state that the proportion of the ends of the 

 conical heater should correspond with the perimeters of the 

 reflector, hence the diameter of the upper end, at the in- 

 tersection of the polygonal plane, should be to that of the 

 lower end as 8 to 6, in order that every part may be 

 acted upon by reflected rays of equal density. This con- 

 dition being fulfilled, the temperature communicated will 

 be perfectly uniform. A short tube passes through the 

 upper head of the heater, through which a thermometer 

 is inserted for measuring the internal temperature. The 

 stem being somewhat less than the bore of the tube, a small 

 opening is formed by which the necessary equilibrium of 

 pressure will be established with the external atmosphere. 

 It should be mentioned that the indications of the thermo- 

 meter during the experiment have been remarkably 

 prompt, the bulb being subjected to the joint influence of 

 radiation and convection. 



The foregoing particulars, it will be found, furnish all 

 necessary data for determining with absolute precision the 

 diffusion of rays acting on the central vessel of the solar 

 pyrometer. But the determination of temperature which 

 uninterrupted solar radiation is capable of transmitting to 

 the polygonal reflector calls for a correct knowledge of 

 atmospheric absorption. Besides, an accurate estimate of 

 the loss of radiant heat attending the reflection of the rays 

 by the mirrors is indispensable. Let us consider these 

 points separately. 



Atmospheric Absorption. — The principal object of 

 conducting the investigation during the summer sol- 

 stice has been the facilities afforded for determining 

 atmospheric absorption, the sun's zenith distance at 

 noon being only 17° 12' at New York. The retardation 

 of the sun's rays in passing through a clear atmosphere 

 obviously depends on the depth penetrated ; hence — 

 neglecting the curvature of the atmospheric limit — the 

 retardation will be as the secants of the zenith distances. 

 Accordingly, an observation of the temper., ture produced 

 by solar radiation at a zenith distance whose secant is 

 twice that of the secant of 17° 12', viz. 6i° 28', determines 

 the minimum atmospheric absorption at New York. The 

 result of observations conducted during a series of years 

 shows that the maximum solar intensity at 17" 12' reaches 

 66°'2 F., while at a zenith distance of 61° 28' it is 52°'5 F. ; 

 hence, minimum atmospheric absorption at New York, 

 during the summer solstice, is 66 0- 2- 52 ,J '5 = i3°7 F., or 

 137 

 tztt = C207 of the sun's radiant energy where the rays 



enter the terrestrial atmosphere. 



In order to determine the loss of energy attending the 



