Oct. 3, 1872] 



NATURE 



459 



December, for the solar radiation, 12'"', by his actinomcter, 

 and, in June 25''"', 56"." It is not my purpose to prove tlie 

 fallacy of M. y Sinobas' actinoinetric observations ; but I think 

 " that, there must be something very singular in his apparatus," 

 since in North America in lat. 40° 42' (the latitude of Madrid is 

 40' 24'), solar intensity at noon during the latter part of June is 

 64' "5 ; wliile when the sky is clear at noon during the latter part 

 of December, the temperature under similar atmospheric con- 

 ditions, reaches 5S'7. But observations made in the morning 

 or evening during the month of June at the hour when the sun's 

 altitude is the same as at itoon in December, show that the 

 intensity of the radiant Iieatinjune is only 53°'oS against 5S"7 

 in December. Actual observations have thus established the 

 fact that for ci'rrciponJnig zenith distance, the temperature pro- 

 duced by the radiant heat when the earth has nearly reached 

 perihelion, is 5S7 — 53'8 = 4°'g itiglier than at midsummer. 

 Referring to the table published in Nature, vol. v. p. 47, it 

 will be seen that, owing to the greater proximity of the sun, the 

 increase of absolute intensity of solar radiation is 5° '88 Fah. 

 during the winter solstice. Pcre Secchi will do well to examine 

 the subject more carefully, and make himself better acquainted 

 with the character of the investigations which had led to an exact 

 determination of the temperature produced by solar radiation. 



The readers of the Coniples Ken Jus who have examined the re- 

 view referred to, ignorant of the contents of the articles in 

 Nature, will be surprised to learn that I have not, as the reviewer 

 asserts, questioned the power of vapour to diminish solar intensity. 

 Having stated the result of numerous observations of the sun's 

 radiant power at corresponding zenith distance, and proved that 

 the temperature during midwinter is higher than at midsummer, 

 I made the following remark in Nature, Nov. 16, 1871 : "In 

 the face of such facts it is idle to contend that the temperature 

 produced by solar r.xdiation under corresponding zenith distance 

 and a ciair skv varies from any other cause than the varying 

 distance between the sun and the earth." It is absurd to suppose 

 that a person having devoted many years to the investigation of 

 solar radiation should deny the retarding influence of vapour, 

 since not one observation in a hundred indicates maximum solar 

 intensity, owing to the presence of vapour in the atmosphere. 



The following brief description of the actinometer will show 

 that there is not, as I'ere Secchi supposes, anything very singular 

 in this apparatus tending to render its indications deceptive. 

 The principal part of the instrument consists of an air-tight 

 cylindrical vessel, the axis of which is directed towards the sun, 

 the upper end being provided with a thin lens covering an aperture 

 of 3in. diameter. The bulb and part of the stem of a mercurial 

 thermometer is inserted through the upper side, at right angles 

 to the axis ; a small air pump being employed for exhaubting the 

 air from the cylindricil vessel. The latter is surrounded by a 

 casing through whicli water is circulated by means of an ordinary 

 force pump and flexible tubes, connected with a capacious 

 cistern containing water kept at a constant temperature of 60^ 

 Fah. The bulb of the thermometer is cylindrical, 3 ins. long, 

 its contents bearing a very small proportion to its convex area. 

 The upper half is coated with lamp black, while the lower half 

 of the bulb is effectually protected against loss of heat from undue 

 radiation. Tlie diminution of energy attending the passage of 

 the sun's rays througli the lens is made good by the concentra- 

 tion elTected by its curvature ; hence the entire energy of the 

 radiant heat will be transmitted to the cylindrical bulb. Tlie 

 inclination of the latter, it should be observed, promotes a rapid 

 upward current of the mercury on the top side, and a correspond- 

 ing downward current on the lower side, thereby rendering the 

 indication prompt and trustworthy. The water in the surround- 

 ing casing being maintained at a constant temperature of 60° Fah. , 

 it will lie evident that the cero of the therraometric scale of the 

 actinometer must correspond with the line which marks sixty 

 degrees on the Fahrenheit scale. It scarcely needs explanation 

 that the height reached by the mercurial column after turning the 

 instrument towards the sun, wiU be due wholly to solar energy, 

 since the radiation of the exhausted vessel towards the bulb of 

 the thermometer is only capable of raising the column to the 

 actinometric zero (60° Fah.) 



The readers of Nature will remember that one of the articles 

 reviewed by Pere .Secchi, the one published in vol. v. pp. 449-452, 

 contained a demonstration accompanied by several diagrams, 

 proving that the radiant heat emitted by the chromosphere and 

 outward strata of the solar envelope is inappreciable at the sur- 

 face of the earth. It will be remembered also that the mode 

 adopted in deciding the question whether the solar atmosphere 



is capable of emitting heat rays of appreciable energy, was that 

 of shutting out the rays from the photosphere, and collecting 

 those from the chromosphere and envelope in the focus of a 

 parabolic reflector. Scarcely any heat being produced notwith- 

 standing the great concentration by the reflector, we proved the 

 fallacy of Pere Secchi's remarkable assumption that the high 

 temperature at the surface of the pliotosphere is caused by 

 radiaiion "received from all the transparent strata of the solar 

 envelope." It is surprising that notwithstanding the complete- 

 ness and positive nature of the demonstration referred to, no 

 allusion whatever is made to the same in the review put forth as 

 a careful examination of the contents of the article under con- 

 sideration. Ignoring the evidence furnished by actual trial, in 

 proof of the extreme feel^leness of the radiating power, the re- 

 viewer proceeds to state " that the outward strata might be less 

 hot, and that the effect which we measure is the aggregate of the 

 quantities of heat which are added, emanating from the various 

 transparent strata." How the outward colder strata cause an 

 elevation of temperature by their radiation towards the solar sur- 

 face, is not explained ; but reference is made to the result of an 

 experiment with three small flames, in support of the assertion 

 that the high temperature of 10,000,000" C, assigned to the 

 surface of the sun, is owing to radiation received from all the 

 transparent strata surrounding the photosphere. The reviewer 

 states : " a very simple experiment, made at my request by P. 

 Provenzali, has shown that, if a heating of 2°-5 can be obtained 

 with one flame, with two flames placed one before the other 

 4°-5 are obtained, with three flames 5°'4 ; a result, which of 

 course could be easily foreseen, for everybody knows that flames 

 are transparent." 



My practical demonstration establishing the feebleness of the 

 radiating power of the matter composing the solar envelope 

 having received no consideration, while the reviewer, in support 

 of his singular theory of solar temperature, points to the result 

 of the rude experiment conducted by Pere Provenzali, I have 

 deemed it necessary to show that transparency of flames is too 

 imperfect to warrant the inferences drawn. 



The accompanying illustration represents an apparatus by 

 means of which the exact degree of transparency of a series of 

 flames has been ascertained. 



Description : b conical vessel open at the top, the bottom com- 

 municating with a cylindrical chamber,/ by an intervening 

 narrow passage, the whole being enclosed in an exterior vessel e 

 charged with water kept at a constant temperature, precisely as 

 in the actinometer. A thermometer is applied near the bottom 

 of the cylindrical chamber, the centre of the bulb coinciding with 

 the prolongation of the axis of the conical vessel. A gas pipe, d, 

 provided with a series of vertical burners, is firmly secured to a 

 table, in a position parallel to the axis of the conical vessel. The 

 burners are provided with caps in order to admit of any desirable 

 number of jets being ignited at one time. When gas of ordin.ary 

 pressure is admitted into the pipe </, t'ne side view of the flanres 

 will be as indicated by the dotted lines at «;, in, the thickness of 

 each flame being nearly 0'20 ins , while the width, shown by the 

 dotted lines n, n, somewhat exceeds 3 inches froai point to point. 

 It will be observed that the prolongation of the axis of the coni- 

 cal vessel upwards passes throagh the central portion of the 

 flames at the point of maximum thickness and intensity. Sup- 

 posing that the instrument (attached to a tabic turning on decli- 

 nation axis within a revolving observatory) is directed towards 

 the sun, it will be evident that all the rays of a Ijeam the section 

 of which corresponds with that of the bulb of the thermometer, 

 will pass through the flames before reaching the said bulb. Now 

 the temperature of the flames at the point pierced by the solar 

 rays, is fully 2000° Fah., while the intensity of the rays does not 

 exceed 60°. The illustrated device enables us to ascertain whether 

 the rays thus entering at a temperature 1,940° lower than that ot 

 the incandescent gas, have their intensity augmented or diminishe 1 

 during the passage through the heated medium. But before ^ve 

 can determine this question, it will be necessary to ascertain what 

 temperature is communicated to the thermometer by the radiant 

 energy of the flames, independent of solar heat. Accordingly, a 

 series of experiments have lieen made, the result of which is re- 

 corded in the annexed table. The nature of the investigation 

 will be readily understood from the following explanation. The 

 instrument being turned away from the sun and the upper flame 

 ni ignited, while the external casing c is kept at a constant tem- 

 perature of 60=, the column of the thermometer at/ slowly rises 

 to 6i°76. The radiant heat, therefore, of a single flame pro- 

 duces a differential temperature of 6176-60= i°76. ''''— 



The 



