MAC 
wheie those unacquainted with tlieir form 
and habits may be easily gratified by a 
sight of them in various stages of growth, 
and bounding before him wdth a vivacity 
and elasticity highly entertaining. See 
Mammalia, Plate IX. fig. 3. 
MACTRA, in natural history, a genus of 
the Vermes Testacea class and order. Ani- 
mal a tethys ; shell bivalve, unequal sided, 
equivalve ; middle tooth of the hinge com- 
plicated, with a small hollow on each side ; 
lateral ones remote and inserted into each 
other. There are twenty-seven species. 
MACULiE, in astronomy, dark spots ap- 
pearing on the luminous faces of the sun, 
moon, and even some of the planets ; in 
which sense they stand coratradistinguished 
from facul®. See Facul^e. 
These spots are most numerous and easily 
observed in the sun. It is not uncommon 
to see them in various forms, magnitudes, 
and numbers, moving over the sun’s disk. 
They were first of all discovered by astrono- 
mer 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 disk ; others to decay and va- 
nish there, or at some distance from his 
Ijmb. a.Their apparent velocities are alw'ays 
greatest over the middle of the disk, and 
gradually slower from thence on each side 
towards the limb. 3. The shape of the 
spots varies according to their position on 
the several parts of the disk : 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 optics. 
By comparing many olrservations of the 
intervals of time in which the spots made 
their levolution, 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 : therefore say, 
as tlie angular motion of 360° -|- 26° 22', is 
to 360° j so is 27 days, 12 hours, 20 mi- 
nutes, to 25 days, 15 hours, 16 minutes ; 
which, therefore, is the time of the sun’s re- 
volution about its axis. 
As to the magnitude of the spots, they 
are very considerable, as will appear if we 
pbserve that some of tliem arc so large as to 
MAC 
be plainly visible to the naked eye : tlui* 
Galileo saw one of them in the year 1612 ; 
and Mr. Martin assures us, that he knew 
two gentlemen that thus viewed them 
several years ago; w'hence he concludes, 
that these spots must therefore subtend, 
at least, an angle of one minute. Now 
the diameter of the earth, if removed to the 
sun, would subtend an angle 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, there- 
fore, 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 su- 
perficies ; whence the surface of the spot is, 
to the surface of the earth, as 9 to 4 ; or as 
2J to 1. Gassendus says, he saw a spot 
whose diameter was equal to ^ of that of 
the sun, and therefore subtended an angle * 
at the eye of 1' 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 substances than solid bo- 
dies, because they lose the appearance of 
solidity in going otfthe disk of the sun : they 
resemble something of the nature of scum 
or scoria, swimming on the surface, which 
are generated and dissolved by causes lit- 
tle known to us : but whatever these solar 
spots are, it is certain they are produced 
from causes very inconstant and irregular ; 
for Scheiner says he frequently saw' fifty at 
once, but for twenty years after scarce any 
appeared. And in the last century the spots 
were very frequent and numerous till the 
year 1741, when, for three years succes- 
sively, very few appeared , and now, since 
the year 1744, they have again appeared as 
usual. 
These maculae are not peculiar to the 
sun, they have, been observed in all the 
planets. Thus Venus was observed to have 
several by Signior Blanchini, in the year 
1726. As in Venus, so in Mars, both dark 
and bright spots have been observed, first 
by Galileo, and afterwards by Cassini, &c. 
Jupiter has had his spots observable ever 
since the invention and use of large teles- 
copes. Saturn, by reason of his great dis- 
tance on one hand, and Mercury, by rca.son 
of his smallness and vicinity to the sun on 
the other, have not as yet had any spots 
discovered on their surfaces, and conse- 
quently nothing in relation to their diurnal 
motions and inclinations qf their axis to the 
planes of their orbits can be known, vyhich 
