REF 



REF 



it, so that it is perpendicular to the centre 

 of the circle; it comes over the index glass, 

 but does not loach it. The other handle 

 is screwed into the handle of the crooked 

 tube, G, so as to be in the same line w'uh 

 the upper handle m ; o, are three da- k 

 glasses, between the index and horizon 

 glasses, turning 1 on a joint, so as to be 

 put out ot" the way when necessary, or 

 any one or two of them can be turned in 

 the line of the telescope to darken the 

 light, more or less, in observations of the 

 Sim ; p, are three other glasses supported 

 by a small pillar behind the horizon glass, 

 which can also be turned brick as n ne- 

 cessary. The telescope is screwed into 

 a brass ring, r, this is supported by a 

 square piece of brass, tapped at the cor- 

 ners, so as to form a screw ; and by turn- 

 ing, s, a nut upon a screw, the telescope 

 can be raised or lowered parallel lo itself; 

 there is also an adjustment, to bring the 

 line of collimation of the telescope to be 

 parallel to the plan of the circle. 



The circle is divided on the silver ring 

 shown in the plan, into 720 parts, each of 

 which answers to a degree ; as this in- 

 strument measures double the angle 

 shown upon the arc, the same as the Had- 

 ley's quadrant, these are subdivided into 

 three, each of which will be twenty mi- 

 nutes. The verniers include fifty-nine of 

 these divisions, and are divided into sixty, 

 the coincidences of these will subdivide 

 each original division of the circle into 

 sixty parts, each equal to twenty seconds. 

 The arm on which the vernier, D, is fixed, 

 has a clamp at its end to fasten it to the 

 circle ; and a fine screw, x, to move it 

 slowly a small quantity after it is clamped. 



We shall now describe the manner of 

 making an observation by this instrument, 

 of the angle between two objects, nearly 

 in the same horizontal plane; we suppose 

 all the adjustments of the instrument to 

 be perfect ; the observer first holds the 

 instrument in his right hand by the han- 

 dle screwed to the lower handle of the 

 tube G, he looks through the telescope, 

 k, and unsilvered part of the horizon 

 glass, h, and directs it to one of the ob- 

 jects which will be in the clotted line, kg t 

 he then turns the index and index glass, 

 b, by its arm D (which must he undamp- 

 ed) until the other object in \he line, y b, is 

 renected from b to h, and from /;, by the 

 silvered part of the glass, into the line k , 

 in which is placed the obsener's eye; lie 

 then clamps the arm, D, and gently turns 

 the screws, x, backwards or forwards, 

 until the reflected image of the object in 

 the line y t and the other object seen 



thiough the telescope, both exactly co- 

 ver one another. The observation is now 

 half made, and the observer reads off" and 

 writes down the degrees, minutes and se- 

 conds, of each vernier, he then inverts 

 the instrument, holding it by the handle, 

 ?n, and directs the telescope to the object, 

 in the line y b, and brings the reflected 

 image of the object, in the line kg, into 

 view, by turning round the index and in- 

 dex glass the same as before ; the obser- 

 vation is then read off and registered. 

 To determine the angle measured, a mean 

 of the products of both observations 

 must be taken; this is the angle between 

 the lines, y b and kg. A small micro- 

 scope, M, in the plan, is used to examine 

 the verniers, and it can be applied to 

 either verniers as required. 



The dark glasses, o p, are only wanted 

 in observing the sun or moon. 



It is evident that by inverting the in- 

 strument, as we have described, the index 

 error is of no consequence, as it will be 

 always more in one observation and less in 

 the other, 



REFLECTION. As the rays of light 

 are reflected by polished surfaces, so it is 

 found that the rays of caloric have the 

 same property. The Swedish chemist 

 Scheele discovered, that the angle of re- 

 flection of the rays of caloric is equal to 

 the angle of incidence. This has been 

 more fully established by Dr. Herschel. 

 Some very interesting experiments were 

 made by Professor Pictet of Geneva, 

 which proved the same thing. 



These experiments were conducted in 

 the following manner. Two concave 

 mirrors of tin, of nine inches focus, were 

 placed at the distance of twelve feet two 

 inches from each other. In the focus of 

 the one was placed the bulb of a thermo- 

 meter, and in that of the other a ball of 

 iron two inches in diameter, which was 

 just heated so as not to be visible in the 

 dark. In the space of six minutes the 

 thermometer rose 22. A similar ef- 

 fect was produced by substituting a light- 

 ed candle in place of the ball of iron. 

 Supposing that both the light and heat 

 acted in the last experiment, he interpos- 

 ed between the two mirrors a plate of 

 glass, with the view of separating the rays 

 of light from those of caloric. The rays of 

 caloric were thus interrupted by the plate 

 of glass, but the rays of light were not 

 perceptibly diminished. In nine minutes 

 the thermometer sunk 14 ; and in seven 

 minutes after the glass was removed, it 

 rose about 12. Me therefore justly con- 

 cluded, that the caloric reflected by the 



