5.] 



♦ KNOWLEDGE ♦ 



harness was removed, the aas turned round, floored his master with 

 a well-planted kick, and then leisurely and systematically kicked 

 him all to pieces. If I had been by, I should not have interfered. 

 Turn and turn about is surely fair play. I saw a man the other 

 day beat his horse till he could strike no more ; it could not stir a 

 too heayy load of sand. Then he shouted to some fellow-brutes to 

 lend him ttco viore horses. Then he turned to contemplate his own 

 beast. I thought he was moved with compunction. Suddenly he 

 roared " Sacr4 cochon ! " and struck his beast O'lain ! Don't talk of 



)iui 



u superiority after that. 



MEAN-TIME SUN-DIALS. 



[1892]— The following hints may be acceptable to those who 

 wish to set up mean-time sun-dials. The construction, though not 

 raathematically exact, is, I think, sufficiently so for all practical 

 purposes. 



I premise that the reader is already acquainted with what has 

 appeared in Knowledge on this subject. 



I would not have the radius of the hour-circle more than six 

 inches, or the shadow of the gnomon will be too indistinct. 



The axis of the gnomon may be a stout steel wire. The waist 

 may be made as thick as is necessary for strength, as this can be 

 allowed for on the hour-circle. I mean to try if plaster-of-Paris 

 will do for the material of the gnomon itself. 



I do not see any special reason for making the two bulges of the 

 gnomon of equal diameter, and I propose, in drawing the curves, 

 to use as simple formulse as possible. 



The ordinates and abscissa) may be drawn from intervals of a 

 week or ten day.s. For the latter multiply six inches by the 

 tangent of the declination, for the former use the radius of the 

 wire (waist) plus the sine of the equation of time. 



The section of the gnomon when drawn might be transferred to a 

 thin sheet of metal and cut out, so as to serve as a gauge. 



I use one dial with two gnomons. The axes of the latter may be 

 made as long as convenient, with their ends fitting into slots, so 

 that they may be changed when required. 



If the gnomon consisted of a cylinder of, say, a tenth of an inch 

 diameter, it would cast a tolerably clear shadow of the same width, 

 aud, at apparent noon, the axis of the shadow give the twelve 

 o'clock mark. It is necessary, therefore, to give the hour circle a 

 movement round the axis of the gnomon so as to set the edge of 

 *he shadow to the exact tine. 



From Christmas to the middle of April, and from the middle of 

 June to the 20th of August, the east side of the shadow has to be 

 used ; at other times the west. When the change of edge has to be 

 made, the hour circle must be revolved the width of the waist of 

 the gnomon. A similar motion is, of course, used (if required) to 

 set the dial to any given standard time instead of local mean-time. 



The gnomon and hour circle must be braced together, and the 

 ' ' of two motions — one round its axis, as just 



,e lowe 



of tl 



a vertical direction to set it for the latitude 

 aiomon (or rather its support) has a hinge, 

 lamped to a graduated arc according to the 



Jatitude. 



When my dial is finished I mean to teat it from time to time by 

 .1 chronometer. 



I see that in my letter 1825 I inadvertently used the expression 

 " sine of half the equation of time " for " sine of the equation of 

 time." MusAFiE. 



OCULAR SPECTRA— THE EETINAL DECOMPOSITION OF 



all probability, lead to some important changes in our theoretical 

 conceptions. 



1. If the sensation of a particular colour be due to a wave of a 

 certain periodicity, how comes it that that wave period can be 

 immediately changed into that of its complementary and back 



within 



,s of ti 



2. If what is usually called the decomposition of light be a 

 veritable decomposition, this decomposition, it is evident, may be 

 effected without refraction. 



3. The decomposition, thus effected, is not sufficiently accounted 

 for by the suggestion that the sequent sensations of colour follow 

 each other according to the strength of the impressions made upon 

 the retina by the respective component vibrations constituting 

 white light, that is to say, the weaker fading first and the stronger 

 last; for, if this were the case, the spectra would preserve the 

 same order of succession whether in a dark or in a light field of 



4. Seeing that identical portions of the field of the retina may 

 experience alternately, and almost simultaneously, opposite sensa- 

 tions of colour, these spectral changes mast be due to the pro- 

 portional relation of the excitement of those portions to their 



The most feasible explanation of the sequent spectra referred to 

 appears to mo to be offered by (4) viz., that those sequent spectra 

 are intervals in the decadence of a strong initial impression on the 

 retina, and that the different sensations of colour experienced are 

 due to differences of degree rather than of kind, just as our different 

 sensations of heat and cold are, and that jast as in the case of 

 temperature the sensation produced by any particular degree 

 may be changed by juxta-relation to some other antecedent or 

 consequent. W. C.i-iT: Thomas. 



A TRICYCLE QUERY. 

 [1894]— Would Mr. Browning oblige by describing a tricycle 

 suitable for a man weighing IGi stone, fifty years old, and, conse- 

 quently, not as active as he might be. A safe, easy-going, and 

 tolerably swift machine (not for racing) is what is wanted. The 

 Globe Lein seems to me to be about what is required. Am doubtful 

 whether a single or convertible machine would be best. 



SlJCTEEN AND A HALF StOSE. 



SWANS FLYING. 

 [1895] — I send the following, as I think some of your readers 

 are doubtless interested in natural history : — 



At 7.40 p.m. I observed three swans [What kind ?], at a fair 

 elevation, flying in a direction from S.W. to N.E. Heads and necks 

 were well stretched out ; but, from the rate at which they travelled, 

 appear at all fatigued. H. A. Miles. 



r. 14. 



OIL-GLAND IN DUCK. 



[189G]— Article " Philosophy of Clothing," p. 115. I think it 

 would be well to finish one heresy before beginning another. I 

 take it that it is well understood by most persons who would be 

 likely to read this paper that down of the duck lies inside the 

 feathers next the skin, thit it encloses a large quantity of air, and 

 that still air is an excellent non-conductor of heat ; but this has 

 nothing to do with the little organ in question, nor is the grease or 

 yolk iu the wool of tlie sheep an analogous illustration, as that 

 applies to the whole surface of the body of the animal; nor does 



with them, an>! :.,.,., 



nthcr in contr:,n ..id.'!. :,-: ilr -■• I'ii.,, I . j- i ,. i . . .1 . ,i i . , ni a 

 dark or in a light' lit'Id of vision. One of the most inten'sting ex- 

 periments which may bo made with thefe spectra is, that of 

 turning the eye either from a light on to a dark field or vice-versa, 

 when the spectrum of the moment will appear of the opposite or 

 complementary colour, assuming its previous hue when the eye 

 is again turned upon a light field. When this remarkable fact 

 .shall have been suflicicntly corsidored in all its bearings, it will, in 



1870— lecturer .-n \i:ii .1 II i.;-:-. .-,; i:ii ;i 



(page 208) as an organ of great dcveiiipn.en; 

 Natatores, by means of which the dense plumage is 

 oiled. And " Anatomy of Vertebrate Animals," I 



