NATURE 



{Feb. 8, 1872 



may be kept accurately in focus during the period re- 

 quired to compl te the observations. The main features 

 ot the apparatus being shown by the illustration, a biief 

 dtscn,Jtion will suffice. The parabolic reflector which con- 

 centrates the rays from the chromosphere (described in 

 the previous article) is placed in the cavity of a conical 

 dish of cast-Ton, secured to the top of a table suspended 

 on two horizontal journals, and revolving on a vertical 

 axle. The latter, slightly taper, turns in a cast-iron socket 

 which is bushed with brass and supported by three legs 

 stepped on a triangular base, resting on friction-rollers. 

 The horizontal journals referred to turn in bearings 

 attached to a rigid bar of wrou^ht-iron situated under the 

 table, firmly secured to the upper end of the vertical axle. 

 The horizontal angular position of the table is adjusted 

 by a screw operated by the small hand-wfheel a, the in- 

 clination being regulated by another screw turned by the 

 hand-wh^el /'. A graduated quadrant, c. is attached to ihe 

 end of the tible in order to afford means of ascertainmg 

 the sun's zenith distance at any moment The indrx d^ 

 which marks the degree of inclination, is stationary, being 

 secured to the rigid bar before described. The rays fro n 

 the photosphere are shut out by a circular disc, f, comoosed 

 of sheet metal turned to ex ict size, and supported by three 

 diagonal rods of steel. These rods are secured to the 

 circumference o^ the conical dish by screws and adjust- 

 able nuts in such a manner that the centre of the disc/ 

 may readily be brought in a direct line with the axis of 

 the reflector. The m.chinism adopted for adjusting 

 the position of the table by the hand-wheels a and b re- 

 quires no rxplanation ; but the device which enables the 

 operator to ascertam when the axis of the reflector is 

 pointed exactly towards the centre of the sun demands 

 particular notice. A shallow cylindric <1 box, ^, provided 

 with a flat lid and open at the bottom, excepting a narrow 

 flange extending round the circumference, is firmly held 

 by two columns secured to the t'lp of the table. A convex 

 lens of 26 inches focus is inserted m the cylindrical b x, 

 the narrow flange inentiuned affording necessary support. 

 The lid is perforated by two openings at right angles, 

 0'05 inch wide, 2'5 inches long, forming a cross, the lens 

 being so adjusted th it its axis passes through the central 

 point of intersection of the cross. The face of the table 

 being turned at right angles to the sun, or nearly so, it 

 will be evident that the rays passing through the perfora- 

 tions and through the lens will produce, at a certain dis- 

 tance, a brilliantly illuminated cross of small size and 

 sharp outline. A piece of ivory, or white paper, on which 

 parallel lines are drawn intersecting each other at right 

 angles, is attached to the top of the table in such a position 

 that the centie of intersection of the said lines coincides 

 with the axis of the lens. This axis being parallel with the 

 line passing through the centre of the disc/and the focus of 

 the reflector, it will be perceived that the operator, in direct- 

 ing the table, has only to bring the illuminated cross within 

 the intersecting parallel lines on the piece of ivory. 

 Ample practice has shown that by this arrangement an 

 attentive person can easily keep the disc / accurately in 

 line with the focus of the reflvCtor and the centre of the 

 sun during any desirable length of tune. The absence 

 o( any perceptible motion of the column of the focal ther- 

 mometer durmg the experiments which have been made 

 furnishes the best evidence tha: the sun's rays have been 

 effectually shut out by the intervening disc, which, it should 

 be remembered, is only large enough to screen the aper- 

 ture of the reflector from the rays projected by the photo- 

 sphere. It may be noticed that actinometric observations 

 cannot be accurately made unless the instiument is 

 attached to a table capable ot being directed in the man- 

 ner described ; nor is it possible to measure the dynamic 

 ener^'y transmitted by solar radiation unless the calori- 

 meter em pi :>) ed for the purpose faces the sun with the 

 sanrie precision as our parabolic reflector, it is worthy of 

 notice that the lightness of the illustrated apparatus ren- 



ders exact adjustment easy, since screws of small diameter 

 and fine pitch may be employed. It only remains to be 

 stated that in order to admit of accurate examination of the 

 spectrum before referred to, the thermometer is removed 

 during investigations which do not relate to temperature, 

 a cylindrical stem of metal, 025 inch diameter, coated with 

 lamp-black, being introduced in its place. 



With reference to the result of recent experiments, it is 

 proper to state that, at the present time, the sun's zenith 

 distance being now nearly 60° at noon, no perceptible 

 heating takes place in the focus of the parabolic reflector. 

 The observations relating to temperature mentioned in 

 the previous article, were made when the zenith distance 

 was only one-third of what it is at present. The conse- 

 quent increase of atmospheric depth, at this time, has 

 completely changed the colour of the spectrum, and ren- 

 dered the same so feeble that its extent cannot be 

 determined. As seen last summer, before the earth had 

 receded far from the aphelion, the termination of the 

 spectrum reached so far down that an addition of 015 inch 

 to the radius of the disc/" would scarcely have shut it out. 

 Now an addition of o'i5 inch to the radius of the disc cor- 

 responds to an angular distance of 9' 45" ; hence, 

 assuming the radius of the photosphere to be 426 300 miles, 

 the depth of the solar atmosphere cannot be less than 

 255 000 miles. And, judging from the appearance at the 

 period referred to, there can be little doubt that a larger 

 and more perfect reflector will enable us to trace the spec- 

 trum still further do>vn. Consequently, a further enlarge- 

 m-.nt of the disc /will be required to extinguish wholly 

 the reflected light from the solar atmosphere. It is 

 reasonable, therefore, to suppose that the depth of the 

 solar atmosphere will ultimately be found to e.xceed very 

 cons'derably \\it foregoing computation. 



It has been suggested regarding the instituted investi- 

 gations of the radiant heat transmitted by the chromo- 

 sphere, that the thermo-electric pile ought to be employed 

 in combination with the p.irabolic reflector. The object 

 of the investigation being simply that of proving by the 

 feebleness of the radiant power transinitted to the surface 

 of the earth that the chromosphere and outer strata of 

 the sun's envelope do not possess radiant energy of 

 sufficient intensity to influence sol ir temperature as sup- 

 posed by Secchi, tests of the suggested extreme nicety 

 are not called for. 



With reference to the effect of increased depth, the 

 small amount of retardation suffered by the rays in 

 passing through the highly attenuated atmosphere of the 

 sun, previously established, shows that the question of 

 solar temperature will not be materially affected, even 

 should it be found that the depth of the envelope is 

 greater than the radius of the photosphere. 



J. Ericsson 



THE RIGIDITY OF THE EARTH 



CIR WILLIAM THOMSON'S views regarding the 

 >-^ rigidity of the earth have been hitherto received in 

 silence by those who entertain different opinions from 

 him ; but it does n^it follow on this account that they 

 regard his position as unassailable. It is more satisfactory 

 to attempt to establish positive results in science, than 

 to criticise the labours of others ; but as Sir William 

 Thomson, by his letter in NATURE for January 18, 

 manifestly invites discussion, I hope I shall be excused 

 for making the following remarks. 



When nearly ten years since 1 saw the abstract in the 

 Proceedings of the Royal Society which he appends to 

 his letter, I resolved to suspend my judgment until I had 

 an opportunity for reading his papers in extcnso. To 

 such of your readers as happen to be interested in this 

 question, and who have not yet seen these publications, I 

 would venture to recommend a similar course. In the 



