574 !$LV)M 



Science, scale D if it is north, will be found the time of its ris- 

 Curiostties j n g above the horizon ; and the degree of declination 

 '" on the scales E,and F, according as it is south or north, 

 '"""^ ' will point out on the horary circle the. time of the star's 

 setting. If the rising of the star is known from obser- 

 vation, bring its declination to the time of its rising on 

 the circle of hours, and the index B will point put the 

 time at which it passed the meridian ; and its declina- 

 tion on the opposite scale will indicate the time when 

 it descends below the horizon. In the same way, from 

 the time of the star's setting, we may determine the time 

 when it rises and comes to the meridian. 



The two exterior circles are added to the astrometer, 

 for the purpose of finding the position of the stars and 

 planets in the heavens. The outermost of these is di- 

 vided into 360 equal parts, and the other, which is a 

 scale of amplitudes, is so formed, that the amplitude of 

 any of the heavenly bodies may be exactly opposite 

 the corresponding degree of declination in the adjacent 

 circle. The degrees of south declination, for instance, 

 in the .latitude of 51, corresponds with an amplitude 

 of 15i 8 , consequently the fifteen degrees of amplitude 

 ,Kiust be nearly opposite to the tenth degree of declina- 

 tion ; so that-by a table of amplitudes, the other points 

 of the scale may be easily determined. The astrome- 

 ter is also furnished with a moveable index M N, which 

 carries at its extremities two vertical sights m n, in a 

 strait line with the centre A. The instrument being 

 thus completed, let it be required to find the planet Sa- 

 turn, when his declination is 15 north, and the time 

 of his southing 3 h 30' in the morning. The times of 

 his rising and setting will be found to be 7 h 15', and 

 10 h 45', and his amplitude 24 north. Then shift the 

 moveable index till the side of it which ^points to the 

 centre is exactly above the 24th degree of the exterior 

 .circle in the north-east quadrant, and when the line 

 A B is placed in the meridian, the two sight holes will 

 be directed to the point of the horizon where Saturn 

 will be seen at 7 h 15', the time of his rising. The same 

 being done in the north-west quadrant, the point of the 

 horizon where the planet sets .will likewise be deter- 

 mined. In the same way the position of the fixed stars, 

 'arid the other planets, may be easily discovered. 



If it is required to find the name, of any particular 

 Star that is observed in the heavens, place the astrome- 

 ter due north and sputh, and when the star is near the 

 horizon, either at, its rising or setting, shift the move- 

 s able index till the two sights point to the star. The 

 ftjde pf the index will then point out, on the exterior 

 circle, -the star's amplitude. With this amplitude enter 

 the third scale from the centre, and find the declina- 

 tion of the star in the second circle. Shift the move- 

 able horary circle till the time at which the observation 

 is made be opposite the star's declination, and the in- 

 dex B will point to th^e time at which it passes the me- 

 ridian. The difference between the tiqpie of the star's 

 southing, and 12 o'clock noon, converted into degrees 

 of the equator, and added to the right ascension of the 

 aiin. if the star comes to the meridian after the sun, but 

 aubstracte^l from it if the star souths before the sun, 

 will give te right ascension of the star. With the 

 right ascensions a^nd declination thus found enter a ta- 

 ble of the right ascensions and declination of thejirin- 

 eipal fixed stars, and you will discover the name of .the 

 star which corresponds with, these numbers. The me- 

 ridian altitudes of the heavenly bodies may always be 

 found by counting the number pf degrees between 

 their declination and the index B, The astrometer 

 may be employed in the solution of various other pro- 

 lems ; .but the application of it to other purposes is 

 iejft to the ingenuity of the young astronomer. 



; N e E. 



CHEMISTRY-. 



The science of chemistry presents such a mass of 

 curious and amusing experiments, that it would be a 

 hopeless task to attempt even to enumerate them in a 

 short article like the present. Most of them too have 

 been so often exhibited in lectures, and so frequently 

 described in popular collections, that the repetition of 

 them here would be unprofitable. We shall, there- 

 fore, confine ourselves to the description of some of the 

 most curious and most recently discovered experiments 

 and instruments, which could not well be introduced 

 into any other part of the work. 



1. On the Revival of the Inscriptions on Coins and Me- 

 dals by Unequal Oxidation. 



It has been long known, though we have not been 

 able to ascertain to whom we owe the discovery, that 

 a coin, from which the inscription and the .figures have 

 been entirely effaced, so as not to present the slightest 

 trace of an impression, may have the inscription and 

 figure partly or wholly restored, by placing it upon a 

 hot iron. .In order to perform this experiment with 

 the fullest effect, the coin employed should be one 

 equally worn down, and in which very little of the 

 metal has been rubbed off the hollow parts by which 

 the letters are surrounded. 



When a, coin of thisikind, or what is still better, a 

 coin on which an illegible trace of the letter still re- 

 mains, is placed upon a heated iron, it will be seen 

 that an oxidation takes place over its whole surface, 

 the film of oxide changing its tint with the intensity 

 or continuance of the heat. The parts, however, 

 where the letters of the inscription had existed, oxidate 

 at a different rate from the surrounding parts, so that 

 these letters exhibit their shape, and become legible in 

 consequence of the fihn of oxide which covers them 

 having a different thickness, and therefore reflecting a 

 different tint from that of the parts adjacent. The 

 tints thus developed sometimes pass through many or- 

 ders of brilliant colours, particularly pink and grent. 

 and settle in a bronze, and sometimes a black tint, 

 resting upon the inscription alone. In some cases the 

 tint left on the trace of the letters is so very faint that 

 it can just be seen, and may be entirely removed by a 

 slight friction of the finger. 



When the experiment is often repeated with the 

 same coin, and the oxidations successively removed after 

 each experiment, the film of oxide continues to dimi- 

 nish, and at last ceases to make its appearance. It re- 

 covers the property, however, in the course of time. 

 When the coin is first placed upon the heated iron, 

 ami consequently, when the oxidation is the greatest, 

 a .considerable smoke rises from the coin, an.d dimi- 

 nishes like the film of oxide by frequent repetition. A 

 coin which had ceased to give out this smoke, smoked 

 slightly after .twelve hours exposure to the air, having 

 been removed from the hot iron at the beginning of 

 that interval, and .replaced upon it at the end of it by 

 a pair of pincers. 



From a great number of experiments I have foupdl 

 that it is always .the raisd parts of the coin, and in 

 modern coins the elevated ledge round the inscription 

 that oxidate first. This ledge, in an English shilling 

 of 1816, began by exhibiting a..brilliant yellow tint be- 

 fore it appeared on any other part of the coin. 



In examining a number of old <;oins, a brilliant red 

 globule, accompanied with a smeil of sulphur, appear- 

 ed on one or two points of the coin ; and sometime* 

 small glpbuies, like those of quicksilver, exuded from 

 the surface. Other coins exhaled a most intolerable 

 smell ; and an Indian pagoda became perfectly black 

 when placed upon the hea.ted iron. 



Such being the genera] facts respecting the oxida- 



