8S4 
THE MOST POWERFUL LOADSTONE EVER KNOWN. 
Tin's gave place in its turn to the quadrant, 
divided into degrees and parts of a degree by 
means ofa radius turning round its centre, in 
wlticb were placed fine pins or sight vanes. 
It wa« with such in«trnitients as these that 
Eratosthenes and Ptolemy attempted the 
measurement of the figure and magnitude of 
the earth, and the determination of the olrli- 
quity of tiie ecliptic. Ptolemy states that the 
distance between the tropics in his time was 
found by such an instiument to be J_.l of the 
whole circumference, tliatis l.iof3C08=47®42 
40 ', and the lialfof this or 23*^ 5l' 20 ' con- 
stitutes what is called the obliquity of the 
ecliptic. 
'J’he accuracy of observations made with the 
qnadiant could not Ire great till the invention 
of the telescope and the vernier or reading 
microscope, 'i'he quadrant, though a good 
instiument with these appendage', and was 
long so n-ed, has, at last, given place almost 
universally to the circle which hy means of 
verniers reading loniid the whole circum- 
ference de-troy by mechanical means, pio- 
balrly, the small incident.il enois inseparable 
fiom mateiials and worlcmanship, iiowever 
excellent Irotli may Ire. “ With all the care 
that could be empioyeil, eriors to the amount 
of 20 or 30 were kmowii to exist in the 
observations ot some of tire continental obser- 
vatories, and even to the amount of fiom 5 ' 
to 10" in those of (Jieenwicli.” Indeed, 
Tiougbton has been beard to affirm that 
a wcdl divided circle of a single foot in 
diameter is imrre to be depended upon than a 
fixed quadrant of the largest construction. In 
aseiies of four olr^ei vations made with tlie six 
inch ciiclesof Kaier ns consti ucted by Robin- 
son, I have never found, under favourable 
circumstances, the eriors to exceed ten or 
fifteen seconds. Now, in the preface to the 
first volume <rf the (jieenwicdi observations, 
published by Maskelynein 1776, be makes the 
following lemarks; 'I'be sun and moon and 
some of the piincipal fixed stars are con- 
stantly observed on the meridian everyday 
when the weather will permit ; and t he exact- 
ness of ihe insiiuments is so gieat, and their 
rectifications so nice, that the place ofany 
lieavenly I ody may always 1 e found by them 
within ten secondsof a degree both in longitude 
anil laiituile, and geneially much nearer.” 
lie then possessed a gie.-.t mural quadiant of 
eight feeliadiui, by Hiid, and we, tlieiefoie, 
see that onr small ciicles of few inches in 
diameter are neaily as accurate as the old 
quadrants ofas many feet, and they approach 
much nearer to perfection than we had any 
reason to anticipate. Such small portable 
circles are consequently very valuable to the 
amateur astronomer, as well as the scierriific 
traveller, since, in the hands of a skilful 
observer, they furnish results highly useiul 
for the improvement ofgeogiaphy, astronomy, 
and iravigalion, while at the same lime llieir 
moderate price enables many to become 
pu I chasers. 
In a letter from Captain Katerof the 2.5lh 
of February, 1831, he remarks: “ the size I 
recommend, and which I use is only 3 inches 
in diameter, and in the latest construction 
has only a vertical circle which can, how- 
ever, be placed in the plane of any two ob- 
jects so as to take the angle between them, 
the whole contained in a box 7 inclir s lomr, 
4| tnclies wide, and 3 deep, so that it really 
deserves the name I originally gave, that of a 
pocket a/imuth and altiiude circle. With 
this little circle I can vet, in one evening, 
my latitude to within 5" of the tMiib by the 
pole star.” Such are liteially the eXfrres- 
sions of the late Capiain Karer, the inventor 
of this insti ument, and the advantages of it to 
scientific travellers are very obvious. 
(To he continued.) 
ON A VERY POWERFUL NATURAL 
MAGNET. 
By Mn. J. Crichton, Gi.asgow. 
To the Editor of the Records of General 
Science, 
Sir,— T he extremely small loadstone which 
belonged to my deceased father, and of which 
you desiie to leceive some details, is perhaps 
the mo't powerful of its kind ever known. 
By the scientific reader it will probably I e 
regarded as a veiy curious and laie piuduc- 
tion of nature, while by some oUiei’s it may bo 
thought to (.'O'sess interest of a (l:ff,-rent 
description. Since no otbei person, now alive, 
knows any tiring of its history, 1 sliall.froiu 
my own immediate knowledge and ciicu.m- 
stanlial recollection, give the best account of 
its origin, which at this lime it is possible to 
put on record. 
In 1772, or the succeeding year, when 
Benjamin Franklin was in Glasgow, he calletl 
on the late Prolesmr Andeison. .Much of 
the conversation which too c place between 
them was on electiicity ; botli were eniliu- 
siasis in this branch of science. Ami, at tlieir 
joint representation, the tliunder-rod, still on 
the ( ollege .steeple here, was erected with a 
view to its protection from the effects of light- 
ning. 
Magneti-m also became the subject of dis- 
cussion, in the cuui'se of wbicli, tim Piofessor 
desired my father, who at that time lived in 
tile Professor’s house as bis mecliani-t, to ex- 
hibit some artificial magnets be bad iu'l finish- 
ed. On this occasion Franklin merit ioned, 
on the anlliorily of his friend Washington, 
thatsome place in Virginia afforded very fine 
loadstones, and added, that on his return to 
Ainetica, he would endeavour to procure a 
specimen and send it to the Professor. 
'J’his was not neglected, for, in 177G. the Pro- 
fessor received the promised mineral, v\hich 
was probat ly brought to France by Dr. 
Fiankliri, '.whence he transmitted it to Glasgow 
as a piesent from Washington himself. 'I'he 
most promising pot lion of the mass was select- 
ed, and my father, then working on his own 
account, was employed toarmit in the most 
appioved manner; but, though this w’as care- 
fully performed, its power was in no way 
remarkable. Several smaller portions of the 
