ALTHIONIC ACID. 



ALTO-RILIEVO. 



250 



BCD, or E B c and B c f. If two lines be parallel, the alternate angle 

 made by a third line with them are equal. 



V 



In algebra, those terms of a proportion are said to be alternate whic' 



]>ar;tte<l from one another by another term; thus, in the pro 



portion 



2 is to 4 as 8 is to 16, 

 2 and 8 are alternate terms, as also 4 and 16. If alternate terms Ix 

 red consecutive, and consecutive terms alternate, the proportioi 

 '.ntimies; thus, 



2 is to 8 as 4 is to 16. 



This proposition is the sixteenth of the fifth Book of Euclid, and is 

 referred to by the Latin word aticrainiilo, or by the English words ' b; 

 alternation,' or ' alternately.' 



ALTHIONIC ACID (HO, C,H 5 S 2 0, ?) is produced when oil o 

 vitriol in great excess is heated with alcohol until olefiant gas begin 

 to be evolved. It has the same composition as sulphovmic acid, bu 



\its compounds differ in crystalline form. It is highly probable tha 

 the althionates are compounds of the sulphovinates and isethionates. 

 ALTIMETER is the name which the Rev. E. L. Berthon gave to < 

 measuring instrument patented by him in 1850. It is a somewha 

 1 complicated contrivance, intended to measure the altitude of the sun 

 \ moon, or a star. There are two glass bulbs, one containing mercury 

 uid the other spirit, and placed in connexion with each other b; 

 il and horizontal tubes. A telescope is attached, through which 

 he object is viewed. The vertical tubes are graduated ; and the 

 ceights at which the two fluids adjust themselves in these tubes, whei 

 the telescope is directed obliquely upwards, is made to indicate th< 

 altitude or angle of elevation of the object viewed, by a particular 

 arrangement of the several parts. 



ALTISSIMO, in Music, is the scale which commences with F, the 

 octave above the fifth line in the treble. 



ALTITUDE, from the Latin ofttw, high, may be rendered by the 

 Knglish word height. This being the case, we should have referred it 

 fc i the English word, if the term were not particularly reserved in 

 astronomy to signify, not the length, but the angle of elevation. Thus 

 if A be the position of a spectator on the earth, and A B the line on the 



horizon, which is drawn towards the point directly under the star s, 

 the angle B A 8 is the altitude of the star. For other less common 

 applications of the term, see HEIGHTS. 



The altitude of the pole is the geographical latitude of the place of 

 observation, and remains the same throughout the twenty-four hours : 

 the altitudes of the stars and sun change with the diurnal motion ; 

 being nothing when they rise and set, and greatest when they are on 

 the meridian. 



The altitude of a star is directly observed at sea with the SEXTANT ; 

 ami the uses which are made of such observations may be seen in the 

 following mathematical propositions, into the proofs of which we 

 cannot enter here. 



1. When the latitude of the place is known, the time of day 

 may be found from one observation of the altitude of the sun or a 

 star ; or conversely, if the time of day be known, the latitude may be 

 found from the observation. 



2. When neither the time nor the latitude is known, both may be 

 found by observing any two altitudes of the sun or a star, and noting 

 tin; time which elapses between the observations ; but it is most con- 

 venient to observe one altitude first, before a star comes to the 

 meridian, and then wait for the time when the same star comes to the 

 same altitude on the other side of the meridian. Or if the latitude be 

 very nearly known, a more accurate approximation may be simply 

 made by the above method. 



'.',. If the star be one of those which never sets, the latitude of the 

 place is the half sum of its greatest and least altitudes. 



In all that precedes, it is supposed that the star is knwcn, that is, 



that its right ascension and declination are known ; and certain cor- 

 rections must be applied to the observed altitude, for which see DIP 

 PARALLAX ; REFRACTION. 



In fixed observatories on land, the altitude of stars, or rather their 

 zenith distances, which are what the altitudes want of ninety degrees, are 

 observed with the mural or the transit circle [CIRCLE, ASTRONOMICAL ; 

 TRANSIT] ; but only when the stars are on the meridian. Let A be 



the place of the observer, c and B the north and south points of his 

 horizon, p the north pole, z the zenith, c p z B the meridian, arcs of 

 which may be made to represent angles at A, and s the star on the 

 meridian, whose altitude, B s, or c s, according as it is south or north 

 of the zenith, or its complement, the zenith distance, z s, has been 

 observed. Then, E being a point in the equator, and P E being equal 

 to z c, both being ninety degrees, take away the common part z P, 

 and E /. is equal to p c, the latitude of the place. And E s, or the 

 star's declination, is E z diminished by z s, if the star passes the 

 meridian above E, or z s diminished by E z, if the star passes below 

 E. If the star passes between the zenith and the pole, the declina- 

 tion E s is the sum of E z and z s. That is, the declination of a 

 star is the difference between its observed zenith distance and the 

 latitude of the place, if the star passes south of the zenith, or the 

 sum of the same quantities, if it passes between the zenith and the 

 pole. In the first case the declination is north, if the latitude be 

 greater than the zenith distance ; south, if the zenith distance be 

 greater than the latitude : in the second case, it is always north. 



In this way, with a number of minute precautions for the sake of 

 accuracy, catalogues of the declinations of stars are formed, by obser- 

 vation of their altitudes, or, which amounts to the same thing, of their 

 zenith distances. 



ALTO, in Music, is the highest natural adult male voice, or counter- 

 tenor, the usual compass of which is, from F the fourth line in the 

 bass, to C the third space on the treble, for example 



ESE 



Alto is also one name of the instrument called in England the tenor, 

 and in Italy the viola. 



ALTO CLEF, in Music, a name of the C clef, when placed on the 

 third line ; more commonly, in England, called the countertenor clef 

 [CLEF.] 



' ALTO-RILIEVO, BASSO-RILIEVO, MEZZO-RILIEVO. The 

 Italian term alto-rilievo is commonly applied to any work of sculpture 

 connected more or less with a plane surface or back-ground, and hi this 

 general sense is opposed to insulated detached figures, or sculpture in 

 the round. In its more particular meaning, alto-rilievo, or high relief, 

 is usually appropriated to figures which are not only rounded to the 

 Full bulk, but have generally some portions of the figures quite 

 detached. Basso-rilievo, low or flat relief, on the other hand, has only 

 i very slight projection from the ground ; and mezzo-rilievo (a style 

 Between the two), although sometimes rounded to a considerable bulk, 

 ias no part entirely unconnected with the plane surface or ground. A 

 more accurate definition of the styles to which these designations refer 

 will result from the explanations that follow. The terms used by the 

 Jreeks and Romans to distinguish these kinds of relief cannot perhaps 

 >e determined with complete accuracy ; and it may be here remarked, 

 hat those writers are mistaken who suppose the word Torctitike 

 TopfvTiicfi) to have been applied by the Greeks exclusively to alto- 

 ilievo, since Heyne, and indeed other writers before him, have proved 

 hat the term was appropriated to carving, and chiefly chasing in metal, 

 n any kind of relief. The Latin word corresponding with it is 

 alatura. The Greeks seem to have employed the term anaglypta to 

 enote works in relief in general ; and the cctypa scaJptura of Pliny 

 xxxvii. 10) also means work in relief. The term glypta (from y\v<pu, 

 n cut into, to holloa! out), with other words formed from the same verb, 

 ppears to denote sculpture in the concave sense, intaglio. Herodotus, 

 i a passage of his second book (cap. 138), where we have little doubt 

 tiat he is speaking of the sunk Egyptian reliefs (which will be men- 

 ioued in another part of this article), couples a word formed from the 

 erb (y\v<txa with the word typus (TOTTOI) : typus itself (perhaps) 

 Iways means a work in relief properly so called. (See Herod, iii. 88 ; 

 Jicero ad Atticum, i. 10.) Italian writers of the time of Vasari, it 

 ppears, used the term mezzo-rilievo for the highest relief, basso-rilievo 

 >r the less prominent, and stiacciato for the flattest or least raised. 

 Vhatever the origin of this kind of sculpture may have been, and 

 here is no doubt of its being very ancient, an idea will be best formed 



