ALT 



ALU 



gods were raised to a great height above 

 the ground ; those appointed for the ter- 

 restrial were almost on a level' with the 

 surface of the earth; and, on the contra- 

 ry, they dug a hole for the altars of the 

 infernal gods. According to Servius, the 

 first were called altaria, the second ane, 

 and the last crobiculi ; but this distinction 

 is not every where observed, for we find 

 in the best authors, the word am, as a ge- 

 neral word, including the altars of celes- 

 tial, infernal, and terrestrial gods. 



Before temples were in use, altars were 

 erected sometimes in groves, sometimes 

 in the highways, and sometimes on the 

 tops of mountains ; and it was a custom 

 to engrave upon them the name, proper 

 ensign, or character of the deity to whom 

 they were consecrated. Thus, St. Paul 

 observed an altar at Athens, with an in- 

 scription To tlie unfcnoiun God. 



In the great temples of ancient Rome, 

 there were ordinarily three altars ; the 

 first was placed in the sanctuary, at the 

 foot of the statue of the Divinity, upon 

 which incense was burnt, and libations of- 

 fered ; the second was before the gate of 

 the temple, and upon it they sacrificed 

 the victims : and the third was a portable 

 altar, upon which were placed the offer- 

 ings and the sacred vessels. 



Besides these uses of the altars, the an- 

 cients swore upon them, and swore by 

 them, in making alliances, confirming 

 treaties of peace, and on other solemn 

 occasions. Altars also served as a place 

 of refuge and sanctuary to all those who 

 fled to them, whatever crime they had 

 committed. 



ALTAR is also used, among Christians, 

 for the communion-table. 



ALTERNATE, in heraldry, is said in 

 respect of the situation of the quarters. 



Thus the first and fourth quarters, and 

 the second and third, are usually of the 

 same nature, and are called alternate 

 quarters. 



ALTERNATION is used for the dif- 

 ferent ways which any number of quanti- 

 ties may be changed, or combined. See 

 COMBINATION. 



ALTHJEA, marsh-mallow, in botany, a 

 genus of plants, with a double calyx, the 

 exterior one being divided into nine seg- 

 ments; the fruit consists of numerous 

 capsules, each containing a single seed. 

 It belongs to the Monodelphia Polyan- 

 dria class and order. There are nine spe- 

 cies. The A. officinalis is perennial, and 

 flowers from July to September. It grows 

 plentifully in salt marshes, and on the 

 banks of rivers and ditches, in many parts 



of England, Holland, France, Italy, Sibe- 

 ria, &c. It has been in great request in 

 every country where medicine has been 

 regularly cultivated. All its parts abound 

 with a glutinous juice, with scarcely any 

 smell or peculiar taste. 



ALTIMETRY, denotes the art of mea- 

 suring altitudes or heights. See MENSU- 

 RATION. 



ALTITUDE, in geometry, one of the 

 three dimensions of body ; being the same 

 with what is otherwise called height. 



Altitude of a figure is the distance of 

 its vertex from its base, or the length of 

 a perpendicular let fall from the vertex 

 to the base. 



Thales is supposed to have been the 

 first person who applied the principles of 

 geometry to the mensuration of altitude : 

 by means of the staff he measured the 

 height of the pyramids of Egypt, making 

 the altitude of the staff and pyramid pro- 

 portional to the length of the shadows. 



ALTITUDE, in optics, is the height of an 

 object above a line, drawn parallel to 

 the horizon from the eye of the observer. 



ALTITUDE of the eye, in perspective, is 

 its perpendicular height above the geo- 

 metrical plane. 



ALTITUDE of a star, &c. in astronomy, 

 is an arch of a vertical circle, intercepted 

 between the stars and the horizon. 



This altitude is either true or apparent, 

 according as it is reckoned from the ra- 

 tional or sensible horizon, and the diffe- 

 rence between these is what, is called by 

 astronomers the parallax of altitude. 

 Near the horizon, this altitude is always 

 increased by means of refraction. 



ALTITUDE of the mercury, in the baro- 

 meter and thermometer, is marked by de- 

 grees or equal divisions, placed by the 

 side of the tube, of those instruments. 

 The altitude of the mercury in the baro- 

 meter, in and about the metropolis, is 

 usually comprised between 28.4 and 30.6 

 inches : in the course of the last seven 

 years it has not varied from these limits 

 more than twice. Turing the same pe- 

 riod, the thermometer in the shade has 

 been rarely higher than 82 or 83, and 

 this seldom more than three or four times 

 in a whole summer, nor often lower than 

 about 8 or 10 below the freezing point. 

 This degree of cold is not common. 



ALTO-RELIEVO. See REMEVO. 



ALUM, in chemistry and the arts, is 

 denominated the sulphate of alumina, 

 though it is not merely a combination of 

 alumina with the sulphuric acid. It pos- 

 sesses the following characters : 1. It has 

 a sweetish astringent taste. 2. It is solu- 



