E for making observa upon any e ss of natural phenom-

! three kinds: niag-.".. for observing the; lena of ter-

_:. - met _ 1. for ol ser-

._• the phenomena t atmospheric changes:?tr nomical. for observations of the heav-Lies. In an astronomical - rvatory it is neeess . there should be a lixed snpt for the inst] tits, and exemption from tremors and t > eric disturbances. To se-first, the instruments are to be tirmly planted :. -tone piers, complete", is I it 1 from all other bases of su] port and from the build-.-. To secure the second, a sii n is to be chosen secluded from ways of travel and business. It is important that the localitv be drv, of equable t - nearly exempt poss i fogs, clouds. Arc, and - reened

- - - is consistent with a free view : ;.. q. The instruments hex t ■ my is founded are the. i"- k for obtaining and keeping " th transit circle, and the mural form - iring the meridian distances of 51 si it lith. There are la -everal incipal =t iments. viz.: the eqnato-. which can be directed to any part

- " ls: the heliometer. for taking the -~ LirE t.t micrometric measurements: and titu le an 1 azimuth circle, for determining

- - ts of a star's place. Every well has so a variety of lesser

" s. psychrometers, ther-

- ; . are important

- "i-;. < >f the more ancient instruments sect the mural quadrant are se. od the transit circle is grading the mural circle in all the lead-?. The American method of r --' - - by means o: txom'"'•- - ■ ' I a novel and elegant kind

°* - - equipments of the

- the efficien-

£.v " - of the practical observer. -

" -rn practical astronos of Tycho Brahe -■tie of Urani _ Copenhagen

' " rz has disappc

- " :nowm Of - . t astro-

:.....astitul the observatorv of Paris is the oldest. Built in 1667-'71 by order of Louis XIV.. and designed by Claude Per-rault, the famous architect of the Louvre, it was an edifice of great magnificence, but ill adapted to the purpose for which it was intended. Domenico Cassini. an Italian, was its first director. Here Picard labored from 1073 till his death about 16S2; and in recent times this institution attained a high degree of efficiency under the directorship of Arago. It is now directed by Leverrier. The royal observatory at Greenwich began operations in 1676, with Flamsteed for astronomer royal. Sir (x. B. Airy, the present incumbent, has held the office since 1S35. The Tuseulan observatory in Copenhagen was built in 1704, for Roenier, the discoverer of the velocity of light-Peter the Great caused an observatory to be erected in 1725 at his capital, and the French stronomer De Lisle was invited to be its director. The emperor Nicholas built another in 1339 at Pulkova, a small town 10 m. S. of St. Petersburg, on a scale of unprecedented magnificence. The cost was about soOO.000, and $50,000 is annually appropriated from the imperial treasury for its maintenance. It is the best endowed and the most perfectly organized of all continental observatories.

Attached to it are a very fine library and workshops for repairs and alterations in the instruments. "VVilhelm Struve. its first director, has _:iven a complete description of this establishment i I>t*cr\ption de Vobserratoire astrono-mique antral de Pull'oica, 2 vols. fol.. St. Petersburg. 1845). It is at present under the manasrement of his son. Otto Struve. The observatory at Dorpat (founded about 1811) was the scene of the elder Struve's researches in sidereal astronomy, and of the no less useful labors and speculations of Madler in the same department. The observatory of Konigsberg (1813), under Bessel. became second to none during the present century for its contributions toward the improvement of every branch of astronomy. The observatory of Berlin (about 1834) is important on account of the labors of Encke. Here the planet Neptune was first seen by Dr. Galle. Sept. 23. 1846. Of the British public establishments of the first class-there are. besides that at Greenwich already mentioned, the Radcliffe observatory at Oxford (1774). under the directorship of the Rev. R. Main since 1859; that at the cape of Good Hope 11821). memorable for the successful re-- rches of Prof. Henderson of Edinburgh in determining the parallax of Alpha Centaury and which was under the direction of Sir R. Maclear from 1833 to 1870, when he was so -ceeded by Mr. E. Stone of Greenwich: that at Cambridge (1824), under Prof. Adams, as successor to Prof. Challis: the royal observatorv of Edinburgh (about 1825), under Prof. Piazzi Smvth since 1*44: and the roval observatory of Dublin (1774) under Mr. R* S. Bale. The university of Oxford has decided (1875) to found a second observatorv.

There are many other European observatories justly fam< including those of Abo. Altona. A~ f>onn.

Bremen. Bre-ia-:.. B: -. Bnda. Floren ttingen. Hamburg, Leipsic. Munich. E Santiago. Gotha. UpsaL and Vienna. English have al-o observatories a: Madras, at forcnerlv at Paramatta; and at Mel-bourne. .Numerous private observe- - in various parts of the British empire have enriched science with man; illiant dis Thus. Lord P.osse ercct-d at Parsonstown. county Louth. Ireland, the most si instrument known. LasselL ful reflector established at Liverpool, was -first to detect a satellit yi Xej : con :s with the Bond- - nbridge. M *s., the honor of the dis ; H seventh in order of * ites t Saturn.

A: the private observatory : - . Bish in Regent's park. London IS< . J. R. '.. has labored since 1^44 with s t« --. To these may be added Admir IS - rvatory at Bedford, now ~ - John Herschel"? late establishment at 1 Gape of Good Hope: and those Messrs rington. Dawes. Cooper, and others. A scope of 25 in. ap-r .fin shed in 1868. is destined for the island : a. A new observatory, under Pro:. Winneeke. w -established in 1874 t the university of Str -burg. An observatory especially for obs tions of the sun is constructing t 1 - rn. A new ot - - nearly 1873 replace the old in Vienna. - 1 he first t used in the United St t - : . stroi purposes was set up in 18 t Yale The first obs ratory building was erected in 1836 at Williams college. Mass.. by Prof. H kins. Two years later the Hudson obs was organized in connection with the W stern Reserve college, Ohio, under Prof. L director. About the sam - observatory at Philadelphia was =t lish which introduced a c". ss rior to anv before emplove 1. The West Point observatory, under Prof. Bar n ival observatory at Washingt q, under?apt soon followed.

The latter is now 1S7-: the superintendence of Re A :airai Dav -In 1874 it was - lied with a refra t r Alvan Clark i having an t glass 26 in. in aperture, and being prol most erful refract _ e in the world. At Georgetown, I>. C. an observatory w. -in 1844, and about the s I that at Cin-cinnati began operations under Prof. Mil with instruments of admirable perf rman The telescope and property of this have been transferred to the university, and a site of four acres for a nr bser has been selected at Mount L k it. near L wood. 6 m. from the city. Gunbridge. establis two 1; furnished with one of the best - in the world. By means of it Messrs. William C and Geors P. Bond added to astronomical - a new - fo^ f the i zone t-.e ■ tore.

n. of red accounts of t tion- satellites of

Saturn. Uran-: Prof. Win lock. T:.k A.. . .

S. P. Lans voted t deal astroE study of sol . Mich., an obs - ry was r

I sea] :

A more recent es servat ry _- , ..... f university. - gifts to wl

Ion ex I $2 In ad-

: - -• * teles p at Xant -Miss ] U won a Em

>ne at Yass _ I

- Mitchell is at ] j.del-phia. by the lat :. :-ks n: at Tus

: it Ch rlest:.. 5 C. 1 y I. :. L -: at Xc~ Y Mr. 1

Mr. Can : &t If - gs. N.

Y.. t y Dr. Eenry Dra] -:. K. J..

Mr. Yan Ars at PL:

Friends: it - ^ I I college, c to tl

Shati : of Bost - 1 in

1S71 teles rture: t B

I - een lil erally en i we 1 by Ed . Litch-

I of Brooklyn. X. Y n as

Litehn - servatory at Chi _ re t in. in apertm man e at : Pr f. Sal - rvaf the Shel - -- Yale eolies two towers i to i an vrkt. ind in tl - w-th

I '•:. ' ' ins tsgivei .. _

In 1S74. ~ - Francisco

>-" for a teles ram-: r an obs - itory of

_ . cd at Cor-

. in If". I r t. B. A. I forirector of the 1 s ratory t - - . o in:v%

" "• OBSIDLL\ JLYD POiHE. two >r trachyt • . riassy. porous tumefied mass

■ ' n is d t heat J

- rnal

. .a certain temperat the L.va is nuid. Many< 1 swch into a:.-- I pumice, which is like natural pumice in proportion to the amount of alkali in the mineral. If the obsidian is pulverized it does not swell, but merely turns brown. Obsidian heated above the point at which pumice is formed melts into a greenish mass, so that obsidian is often said to be melted pumice; but the obsidian may probably be formed without passing through the phase of pumice. The following table by Abich gives the analysis of two obsidians and two lavas: 1, obsidian from Teneriffe, sp. gr. 2.528; 2, pumice from Teneriffe, sp. gr. 2.477; 3, obsidian from Lipari, sp. gr. 2.370; 4, pumice from Lipari, sp. gr. 2.77:

CONSTITUENTS.

1.

2.

3.

4.

Silica

61.13

62.25

74.05

73.70

Alumina

19.05

16.43

12 97

12.27

Ferric oxide

4.22

4.26

2.73

2.31

Manganic oxide___

0.33

0.23

Lime..............

0.59

0.62

0.12

0 65

Magnesia

0.19

0.79

0.28

0.29

Soda

10.63

11.25

4.15

4.52

Potash...........

3.50

2.97

5.11

4.73

Chlorine...........

0.30

0.53

0.31

0.31

Water.............

0.04

0.22

1.22

Total..........

100.03

99.33

99.94

100.00

The characteristics of obsidian are its glassy lustre, susceptibility to high polish, and hardness, sufficient to scratch glass. The Greeks called it AlOog, as is supposed by some from, sight, in allusion to its translucence. Pliny derives its name from Obsidius, who is said to have brought it from Ethiopia. It was used by the ancients for mirrors, and for various ornamental purposes. The Mexicans used it, under the name of itzli, for knives, razors, and serrated weapons and implements. The pointed fragments were made into arrows. The stone is much used for ornamental purposes, particularly as mourning jewelry, but from its brittleness requires to be worked with great care. The iridescent variety, which has a peculiar greenish yellow color, and commands a high price, is sometimes cut in cabochon and set in rings. The colors of obsidian are numerous, but each specimen commonly has but one shade. The characteristics of pumice are sponginess and lightness, so that, although the specific gravity of the material itself is as great as that of obsidian, it is often bulky enough to float on water. It is of grayish shades, passing into yellow and brown. It is employed in the arts, pulverized as a polishing material, and in the lump for grinding and smoothing surfaces.

Its chief source in commerce is Campo Bianco, one of the Lipari islands, where it forms a hill nearly 1,000 ft. high.