OBSERVATORY. 



the equation of time being applied, will 

 show the time of mean noon for that 

 day, by which the clock may be ad- 

 justed. 



The Equal Altitude Instrument, is an in- 

 strument that is used to observe a celes- 

 tial object when it has the same altitude 

 on both the east and west sides of the 

 meridian, or in the morning and after- 

 noon. It principally consists of a tele- 

 scope about thirty inches long, fixed to a 

 sextantal or semicircular divided arch, the 

 centre of which is fixed to a long vertical 

 axis. 



The Equatorial or Portable Observatory, 

 an instrument designed to answer a num- 

 i>er of useful purposes in practical astro- 

 jiomy, independent of any particular ob- 

 servatory. It may be made use of in any 

 steady room or place, and performs most 

 of the useful problems in the science. 

 The following is a description of one 

 lately invented by Mr. Ramsden, from 

 whom it has received the name of the 

 Universal Equatorial. The principal parts 

 of this instrument (fig. 3.) are, 1. The 

 azimuth or horizontal circle, A, which re- 

 presents the horizon of the place, and 

 moves on an axis, B, called the vertical 

 axis. 2. The equatorial or hour circle, 

 C, representing the equator, placed at 

 right angles to the polar axis, D, or the 

 axis of the earth, upon which it moves. 

 3. The semicircle of declination, E, on 

 which the telescope is placed, and mov- 

 ing on the axis of declination, or the axis 

 of motion of the line of collimation, F. 4. 

 The telescope, which is an achromatic re- 

 fractor with a triple object gfass, whose 

 focal distance is 17 inches, and aperture 

 2.45 inches, and furnished with six differ- 

 ent eye-tubes; so that its magnifying 

 powers extend from 44 to 168. The 

 telescope in this equatorial may be 

 brought parallel to the polar axis, as in 

 the figure, so as to point to the pole star 

 in any part of its diurnal revolution ; and 

 thus it has been observed near noon, 

 when the sun has shone very bright. 5. 

 The apparatus for correcting the error in 

 altitude occasioned by refraction, which is 

 applied to the eye-end of the telescope, 

 and consists of a slide, G, moving in a 

 groove or dove-tail, and carrying the se- 

 veral eye-tubes of the telescope, on which 

 slide there is an index corresponding to 

 five small divisions engraved on the dove- 

 tail; a very small circle, called the re- 

 fraction circle, H, moveable by a finger 

 screw at the extremity of the eye-end of 

 the telescope ; which circle is divided 

 into half minutes, one entire revolution 



of it being equal to 3' 18", and by its mo- 

 tion raises the centre of the cross hairs on 

 a circle of altitude ; and likewise a quad- 

 rant, I, of 1 inch radius, with divisions 

 on each side, one expressing the degree 

 of altitude of the object viewed, and the 

 other expressing the minutes and se- 

 conds of error occasioned by refraction, 

 corresponding to that degree of altitude ; 

 to this quadrant is joined a small round 

 level, K, which is adjusted partly by the 

 pinion that turns the whole of this appa- 

 ratus, and partly by the index of the quad- 

 rant ; for which purpose the refraction 

 circle is set to the same minute, &c. 

 which the index points to on the limb of 

 the quadrant ; and if the minute, &c. giv- 

 en by the quadrant exceed the 3' 18" 

 contained in one entire revolution of the 

 refraction circle, this must be set to the 

 excess above one or more of its entire re- 

 volutions ; then the centre of the cross 

 hairs will appear to be raised on a circle 

 of altitude to the additional height which 

 the error of refraction will occasion at 

 that altitude. This instrument stands on 

 three feet, L, distant from each other 14.4 

 inches ; and when all the parts are hori- 

 zontal, is about 29 inches high: the weight 

 of the equatorial and apparatus is only 

 59$. avoirdupoise, which are contained 

 in a mahogany case. 



The principal adjustment in this in- 

 strument is that of making the line of col- 

 Emation to describe a portion of an hour- 

 circle in the heavens ; in order to which, 

 the azimuth circle must be truly level, 

 the line of collimation, or some corres- 

 ponding line, represented by the small 

 brass rod M, parallel to it, must be per- 

 pendicular to the axis of its own proper 

 motion ; and this last axis must be per- 

 pendicular to the polar axis ; on the brass 

 rod, M, there is occasionally placed a 

 hanging-level, N, the use of which wiH 

 appear in the following adjustments : the 

 azimuth circle may be made level, by 

 turning the instrument till one of the le- 

 vels is parallel to an imaginary line join- 

 ing two of the feet screws ; then adjust 

 that level with these two feet screws ; 

 turn the circle half round, i. e. 180 ; and 

 if the bubble be not then right, correct 

 half the error by the screw belonging to 

 the level, and the half error by the two 

 foot screws ; repeat this till the bubble 

 comes right; then turn the circle 90 

 from the two former positions, and set the 

 bubble right, if it be wrong, by the foot 

 screw at the end of the level ; when this 

 is done, adjust the other level by its own 

 screw, and the azimuth circle will be 



