res 



SPECULUM. 



SPHERE. 



;ce 



rotation of the speculum tends to lengthen the focus of the exterior 

 zones of its surface. There is also an excentric motion, which aug- 

 ments to any required extent the circumferential action. The figure 

 depends on four things, namely, the radius of the primary, that of the 

 excentric, and the angular velocities of it and the speculum. In 

 polishing a six-feet speculum, the number of strokes is eight per minute; 

 and for smaller ones, inversely as their diameter. The polishing of a 

 six-feet speculum requires five hours. The figure of the speculum was 

 tested in Lord Rosse's process, during the grinding and polishing, by 

 observing the reflection of a watch-dial placed perpendicularly over 

 the speculum, at a height of about 90 feet. By carefully watching the 

 image of this dial, the adjustment of the length of stroke could be made 

 with such accuracy that the three-feet speculum, with its whole 

 aperture, was thrown perceptibly out of focus by a motion of the eye- 

 piece amounting to less than the thirtieth of an inch. During the 

 polishing, it was found necessary, in order to prevent irregular expan- 

 sion of the speculum, to maintain a uniform temperature in the 

 polishing-room ; and it was also necessary to have a certain degree of 

 moisture in the air, that the wet polishing-powder should dry at the 

 proper rate. When the air was too damp, the polishing was not 

 attempted ; if it were too dry, a jet of steam was introduced. Instead 

 of water, ammonia soap (or common soap treated with ammonia) was 

 sometimes used with the polishing-powder, as this was found to dry 

 more rapidly. 



A more complete idea of the arrangements for grinding and polishing 

 specxila may be formed from Mr. Lassell's arrangement, as improved by 

 Mr. Nasmyth, and represented in the figure (col. 704), the object being to 

 imitate as closely as possible those evolutions of the hand by which Mr. 

 Lassell had been accustomed to produce perfect surfaces on smaller 

 specula. F L represent fast and loose pulleys for conveying the power, and 

 transmitting it by the endless screw on the shaft, A, to the wheel c. 

 The spindle, K, of this wheel has attached to it a crank or arm, I, which 

 carries a pinion, J, and causes the pinion to revalve round the toothed 

 circumference of the wheel H, which wheel 11, being fixed to a bracket, 

 causes the pinion J to revolve with as many turns as its circumference 

 is less than that of the wheel H, or five to one. The spindle of the 

 pinion J has a wheel, K, fixed to it at its lower end, which wheel n, in 

 like manner, conveys motion to the pinion L, which works on an 

 adjustable centre-pin ; and as the T-groove in which the centre-pin of 

 L works is radial to the centre of the wheel H, this pinion may be set 

 to any degree of excentricity, and yet be in gear with H. The pinion L 

 baa also a cross-crank, M, attached to its under side, which, having its 

 crank-pin, >', also sliding in a T-groove, it may be set to and fixed at 

 any degree of excentricity ; so that by these two excentric movements 

 we have the means of giving to the pin u any compound motion 

 required. The polisher is of wood or other suitable material, coated 

 with pitch, and divided into squares. 



This polisher is free to move upon the pin H, while N causes the 

 polisher to slide over the surface of the speculum s with a motion not 

 like that represented in the figure contained in Holtzapffel's ' Mechani- 

 cal Manipulation,' where the curves are all re-entering, but rather as in 

 the following figure, where the curves do not re-enter. In order that 



every part of the surface of the speculum may continually change its 

 position, with respect to the movements of the polisher, a slow revolving 

 motion is given to it by an tndleas screw on B working into the teeth 

 of the wheel I), which forms the base on which the speculum rests. It 

 ARTS AND SCI. DIV. VOL. VII. 



rests, in fact, on nine equilibrium points in the cell in which it is to rest 

 when actually in the telescope, so as to incur no'risk of distortion. " By 

 means of this arrangement," says Mr. Nasmyth, " a speculum having a 

 decidedly hyperbolic figure may be corrected, and brought to a perfect 

 parabola, or to a spherical curve, or the same may be done in the 

 reverse order, at pleasure." In Lord Rosse's arrangement, the polisher 

 is traversed over the speculum with reciprocating longitudinal motion ; 

 and in Mr. Lassell's, the polisher has a continuous epitrochoidal motion, 

 the path of which is dependent upon the adjustments of L and M. The 

 polisher is made of two thicknesses of pine-wood, with the grain crossed. 

 This, from its lightness, does not require to be counterpoised, as in 

 Rosse's ; and, apparently from its being sufficiently yielding to accom- 

 modate itself somewhat to the form of the speculum, a single coating 

 of pitch was found sufficient, and the polishing was completed with 

 wet powder. The value of the two-feet speculum thus produced is 

 shown by Mr. Lassell's discovery of the satellite of Neptune, an eighth 

 satellite of Saturn, and the re-observation of the satellites of Uranus, 

 which had not been seen since their announcement by Sir W. Herschel. 

 Mr. Lassell's polisher has served as a type to other skilful mechanicians, 

 such as Mr. De la Rue, Mr. Grubb, of Dublin, and Mr. Lassell himself 

 has introduced some modifications. 



Further details respecting specula belong to TELESCOPE; but we may 

 mention the proposal, if not introduction, of silver as a substitute for 

 speculum metal. It reflects 0'91 of the incident light, while speculum 

 metal only reflects 0'67. It is, on the other hand, liable to tarnish, 

 and there are difficulties in the way of figuring and polishing it. 

 M. Foucault, however, has formed a speculum of glass, figured and 

 polished to a true parabola, and deposited a thin but uniform film of 

 silver on its surface, by means of Drayton's process, in which a solution 

 of nitrate of silver is reduced by means of oil of cassia. The exterior 

 surface of the glass speculum is parabolic, but has no lustre. That 

 property may be imparted to it in a high degree without any sensible 

 change of figure, by means of light friction with wash-leather and a little 

 peroxide of iron. In case of tarnish, the polish may be renewed by 

 the same process, and often be repeated before the silvering requires to 

 be renewed. 



SPECULUM METAL. [BRONZE.] 



SPEISS. [COBALT.] 



SPELTER. [Zisc.J 



SPERMACETI, or CETACEUM, a fatty material, obtained from 

 the Physeter macrocephalus, (Oatodons macrocephalus of Beale) 

 [CETACEA, in NAT. HIST. DIV.], a species of whale, generally met with 

 in the South Seas, but also on the coast of Greenland, and occasionally 

 stranded on the coasts of Britain. When purified it is called ccline. 

 It is also soluble in ether, and volatile and fixed oils. It has a 

 white, pearly, or silky appearance, considerable tenacity, but may 

 be broken into mica-like scales, with a smooth or fatty feel, slight 

 fish-like odour, and mild mawkish taste. Its specific gravity is '943 ; 

 it melts at 112, and when a lighted body is applied to it, it burns with 

 a clear flame. 



Sulphuric is the only acid which dissolves it. It is only partially 

 dissolved by the fixed alkalies, and is with difficulty saponifiable. Hot 

 caustic ammonia forms with it an emulsion, which is not decomposed 

 on cooling. 



Long exposure to the air renders it rancid ; it may be again purified 

 by washing in a warm ley of potass. It should be protected from air 

 and light. 



A hundred parts of spermaceti consist of sixty parts of margaric and 

 oleic acids, forty parts of ethal, and 0'9 parts of a yellow extractiform 

 substance. The ultimate composition of cetine seems to be carbon, 

 81 ; hydrogen, 12 ; oxygen, 5. 



Spermaceti possesses the properties common to fatty matters. It is 

 bland and demulcent, with considerable nutritive qualities, when taken 

 internally. It was formerly much used in colds and coughs, united 

 with mucilage or syrup, to shield the throat from the irritation of the 

 air, also in dysentery. Triturated with sugar-candy, and having warm 

 milk added to it, it is a mild nutrient article, fit for children or old 

 persons. It is however now chiefly employed externally as an ingre- 

 dient in ointments and cerates. It is also largely used to form candles, 

 and to burn in lamps. 



SPHERE, or GLOBE, a solid body, the surface of which is every- 

 where equally distant from a given point or centre within it. This 

 distance of each point from the centre is called the radius. In the 

 article MENSURATION will be found the formula: which connect the 

 surface and solidity of a sphere with the radius : we shall here add 

 that the weight of a sphere of pure water is found in ounces avoirdu- 

 pois, by multiplying the cube of the number of inches in the radius by 

 2'417l ; and in pounds avoirdupois by multiplying the cube of the 

 number of feet in the radius by 261 '05. These results multiplied by 

 the specific gravity give the weight of a sphere of any other substance. 



A section made by a sphere and plane is always a circle. When the 

 cutting plane passes through the centre of the sphere, this proposition 

 is obvious from the definition of a circle. When the plane does not 

 pass through the centre the assertion follows so soon as it is shown 

 that a plane curve having all its points equidistant from a given point 

 not in Ike plane is a circle. A section passing through the centre is 

 called a yreat circle, and one which does not pass through the centre a 

 small circle. These terms are incorrect, since a small circle may be in 



7. 7, 



