MAG 



MAC 



feet are wholly useless, and it proceeds 

 by leaping on its hind feet, which it will 

 do to the distance of fourteen or sixteen 

 ieet, and with bounds so rapid in succes- 

 sion, that it exceeds in swiftness a com- 

 mon dog 1 . Kangaroos possess the faculty 

 of separating- at pleasure the two front 

 teeth of their lower jaw ; and the female 

 is furnished vvitl^ a pouch in the abdo- 

 men, of extraordinary depth, in which are 

 placed two teats. But one young one 

 is produced at a time, which, when first 

 observed in the pouch, after its birth, is 

 scarcely more than an inch in length, but 

 grows to a considerable size in this na- 

 tural receptacle before it quits it, and 

 frequently recurs to it for warmth and 

 security after its first dislodgment from 

 it. This animal is in this striking 1 cir- 

 cumstance allied to the opossum genus, 

 ::uder which Gmelin ranks it, but it dif- 

 fers from the opossum materially iu re- 

 spect to the structure of the teeth. Inks 

 general appearance it strongly resembles 

 (be jerboa. It was the only quadruped 

 which Australasia supplied to the English 

 colonists for food. It has been not only 

 imported into England, but has repeated- 

 ly bred in that country, and may be con- 

 sidered as now naturalized ; and though 

 not apparently convertible to any impor- 

 tant service, exhibits a very interesting- 

 variety to the observer of nature. Many 

 of these animals are kept in the royal 

 premises at l\ew, where those unacquaint- 

 ed with their form and habits may be ea- 

 sily gratified by a sight of them in various 

 stages of growth, and bounding 1 before 

 him with a vivacity and elasticity highly 

 entertaining. See Mammalia, Plate IX. 



fig. 3. 



MACTRA, in natural history, a genus 

 of the Yermes Testacea class and order. 

 Animal a tethys ; shell bivalve, unequal 

 sided, equivafvc; middle tooth of the 

 hinge complicated, with a small hollow- 

 on each side ; lateral ones remote and in- 

 serted into each other. There are twenty- 

 seven species. 



MACULJE, in astronomy, dark spots 

 appearing' on the luminous faces of the 

 sun, moon, and even some of the planets ; 

 in which sense they stand contradistin- 

 guished from faculse. See FACUL.T;. 



These spots are most numerous and 

 ou.sily observed in the sun. It is not un- 

 common to see them in various forms, 

 magnitudes, and numbers, moving over 

 the sun's disc. They were first of all 

 discovered by astronomer Galileo, in the 

 year 1610, soon after he had finished his 

 new-invented telescope. It has been 

 supposed that these spots adhere to, or 



float upon, the surface of the sun, for the 

 following reasons. 1. Many of them are 

 observed to break out near the middle 

 of the sun's disc; others to decay and 

 vanish there, or at some distance from his 

 limb. 2. Their apparent velocities are 

 always greatest over the middle of the 

 disc, and gradually slower from thence 

 on each si4e towards the limb. 3. The 

 shape of the spots varies according to 

 their position on the several parts of the 

 disc : those which are round and broad 

 in the middle, grow oblong and slender 

 as they approach the limb, according as 

 they ought to appear by the rules of op- 

 tics. 



By comparing many observations of the 

 intervals of time in which the spots made 

 their revolution, by Galileo, Cassini, 

 Scheiner, Hevelius, Dr. Halley, Dr. Der- 

 ham, and others, it is found that 27 days, 

 12 hours, 20 minutes, is the measure of 

 one of them at a mean ; but in this time 

 the earth describes the angular motion 

 of 26 22', about the sun's centre : there- 

 fore say, as the angular motion of 360 -j- 

 26 22 V , is to 360 ; so is 27 days, 12 

 hours, 20 minutes, to 25 days, 15 hours, 

 16 minutes ; which, therefore, is the time 

 of the sun's revolution about its axis. 



As to the magnitude of the spots, they 

 are very considerable, as will appear if we 

 observe that some of them are so large as 

 to be plainly visible to the naked eye : 

 thus Galileo saw one of them in the year 

 1612 ; and Mr. Martin assures us, tha't he 

 knew two gentlemen that thus viewed 

 them several years ago ; whence he con- 

 cludes, that these spots must therefore 

 subtend, at least, an angle of one minute. 

 Xow the diameter of the earth, if removed 

 to the sun, would subtend an angie of 

 but 20" ; so that the diameter of a spot, 

 just visible to the naked eye, is, to the 

 diameter of the earth, as 60 to 20, or as 

 3 to 1 ; and, therefore, the surface of the 

 spot, if circular, to a great circle of the 

 earth, is as 9 to 1 ; but 4 great circles 

 are equal to the earth's superficies ; 

 whence the surface of the spot is, to the 

 surface of the earth, as 9 to 4 ; or as 2 

 to 1. Gassenclus says, he saw a spot 

 whose diameter was equal to % of that 

 of the sun, and therefore subtended an 

 angle at the eye of If 30" ; its surface 

 must have been five times larger than the 

 surface of the whole earth. What these 

 spots are, it is presumed, nobody can tell ; 

 but they seem to be rather thin sub- 

 stances than solid bodies, because they 

 lose the appearance of solidity in going 

 off the disc of the sun: they resemble 

 something of the nature of scum or scoria. 



