t *** 3 

XVII. A Letter from the Secretary of the 
Royal Academy of Sciences in Sweden, to 
Cromwell Mortimer, M, D. et R. S. Sec . 
concerning the variation of the magnetic 
needle. 

Celeberrimo Domino Dodtori, et Societatis Regiae 
Londinenfis Secretario, Cromwello Mortimer, 
S. P. D. Petrus Wargentin, Acad. Reg. Scient. 
Suecicae Secretarius. 

Head Feb. 21 . ✓'"'v BUT ante paucos menfes lecretarius 
l 75°- Academiae Regiae Scientiarum Sue¬ 

cicae, vir in mathematicis fcientiis verfatiffimus, D. 
Petrus Elvius: cui, ex decreto academiae, ego mox 
fuffedtus fecretarius, munerjs mei efie judicavi, com- 
mercium literarium cum exteris focietatibus, acade- 
niiis, et viris eruditis, inftituere, cum perfualiffimus 
fim ejufmodi literatorum commercia plurimum ad 
fcientiarum incrementum facere. 

** Ut aliquid ad fcientias pertinens tibi imper- 
tiam, paucis narrabo de obfervatis a me nuper 
variantibus quotidie paullulum, fed faepe admodum 
turbatis, declinationibus acus magneticae. 

Halleius veftras dudum fufpicatus eft, efle quod- 
dam inter lumen boreale et acum magneticam com- 
mercium. Id certiflimis experimentis et obfervationi- 
bus evicerunt jam ante aliquot annos Celfius atque 
Hiorterus, aftronomi apud nos, dum viverent, cele- 
bres, qui faepiflime animadverterunt, acum magno- 
pere turbatam atque inquietam effe, quoties lumen 

boreale 


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t»7] 

boreale ad zenit, vel ad plagam coeli meridionalem 
afcendit, ita quidem, ut declinatio videretur fequi 
motum luminis, et intra pauca temporis minuta to- 
tos tres et quatuor gradus aliquando variare. Res 
fide major mihi initio vifa eft. Meis oculis tam mi- 
rum phenomenon notare cupiebam. Cum itaque 
mihi traderetur acus, pedem Suecanum longitudine 
aequans, ab opifice noftro ingeniofiflimo D. Ekftrom 
confedta, agiliffima •, mox, ineunte Februario hujus 
anni, coepi annotare illius declinationes j quas ftatirn 
quotidie variantes deprehendi, prout Grahamus, 
Celfius, etc. an tea obfervaverant, ea videlicet lege, ut 
acus ab hora feptima matutina ad fecundam poft me¬ 
ridiem, ab oriente ad occidentem magis magifque dif- 
eedat, interdum tertiam vel quartam partem unius 
gradus. Poft horam fecundam iterum revertitur ad 
odtavam vefpertinam, ufquedum eundem fere fitum 
attigerit, quem hora ocftava matutina. Per totam 
nodtem fere quieta effe folet, faltem non nifi parum 
circa mediam nodtem abit ad occidentem, mox in¬ 
eunte mane reditura. Hec diurna variatio nunquam 
fallit, fed conftans et fere regularis eft, nifi lumen 
boreale impediat. 

Cum acus hoc modo, a die 6 Februarii ad 15“ 
circa feptimum gradum declinationis * occidentals 
vagata eflet quotidie, eluxit, die 15 0 , lumen boreale, 
non tamen admodum vividum. Magna cum volup- 
tate percepi, acum mox affici, ut intra 10 temporis 

minuta. 


* Haec declinatio non eft vera et media hoc tempore Holmiae, 
fed aliquanto minor vera. At hac occafione non qusefivi veram de- 
clinationem* fed ejus tantum variationem, 

2 




[ 128 ] 

minuta, circa horam decitnam vefpertinam, abiret 
20' ad occafum, et intra alia decern minuta rediret et 
difcederet 37' ad ortum. Ceflante lumine acquievit 
acus. Poftero die infignis contigit turbatio, ideoque 
iplas obfervationes citare non ingratum tibi efle judico, 
pro tota ifta die. 


Tempus 

Declinat ac. 

Tempus 

Declin. acus* 

* 

/ 

Q f 

h 

t 

0 

1 

8 

0 a.m. 7 0 

10 

56 p. m. 

7 

1 

10 

0 

7 4 

1 I 

6 

6 

2 5 

12 

0 

7 10 

31 

10 

1 

5 i 

2 

0 p. m 

• 7 if 

II 

1 9 

6 

43 

4 

0 

7 11 

II 

22 

6 

26 

8 

0 

7 2 

II 

26 

6 

42 

9 

0 

6 50 

II 

37 

5 

23 

10 

0 

6 8 

H 

45 

5 

0 

30 

5 

5 3 i 

II 

58 

4 

35 

IO 

8 

5 47 

12 

0 

5 

0 

16 

15 

5 *9 

12 

i 5 

6 

3 ° 

IO 

3 ° 

6 0 

12 

27 

6 

22 

30 

4b 

72 6 

12 

3f 

6 

55 


Per totam hanc nodlem vix aliquo moraento quie- 
vit acus; fed, omnibus aliis rebus quietis, me folo 
tacitis paffibus acum invifente, nullo ferro admoto, 
vagahatur hinc inde quad vertigine correpta. Lumen 
bo reale hac nodte fuit in plaga raeridionali fplendi- 
dumet vivaciffimum, interdum per totum ccelum fe 
rapidiffimo motu diffundens: fed ego intentus acui, 
non fatis luminis apparentias obfervare potui. Sequen- 
tibus diebus admodum quieta manfit acus, ut et va~ 
riationes diurnge folito minores fuerint. At 'die 28 

Februarii, 



[ I2 9 ] 

Februarii, novo erumpente lumine boreali infigniore. 
Sentiit id acus, quae ccepit vacillare hora poll meri¬ 
diem quarta, foie adhuc fplendente: unde intellexi 
nos proxima nodte vifuros lumen boreale. Nec fefel- 
lit eventus. At locus non permittit ipfas hue tran¬ 
scribers obfervationes: fufficit dixilfe, quod vacillaverit 
acus inter 6° 50' and 9® 1'. Per totum menfem Mar- 
tium nihil praeter confuetas diurnas digrefliones un« 
quam animadverti, ne 6° quidem, licet lumen boreale 
turn confpiceretur, fed debile et quietum prope ho- 
rizontem borealem. 

At die fecunda Aprilis, ruptis induciis, rurfumex- 
arfit lumen, acui infeftans, idque per duos integros 
dies, die nodtuque pariter, quantum ex acu cognovij 
nam ilia continuis agitata motibus fuit, licet lumen 
non nifi. nodtu obfervari poffet. En praecipuas ob¬ 
fervationes. 


April a, 


April 3, 


Tempus 

decl. 

ac. 

h 

1 

0 

i 

2 

40 p. m. — —— 

7 

7 

4 

20 . .—• 

7 

10 

5 

22 —— —- 

7 

21 

10 

31 - —— —■ 

5 

35 

10 

55 - - 

5 

57 

11 

34 “ *— 

6 

2 7 

11 

jT2 —• ... 

6 

0 

1 Z 

3 -. — 

4 

56 

12 

8 — — *— 

5 

2 7 

1 2 

18 — —- —- 

6 

34 

12 

21 — —. <— 

6 

18 

12 

28 — - 

6 

37 

12 

45 — —— • ~ 

6 

2 2 

7 

0 a . m. ——* 

7 

t 


R 

April 



[ ! 30 1 


Tempus 


April 3, 


April 4, 


10 

if 

10 

49 

3 

30 p. m. 

4 

43 p . m. 

4 

49 — 

f 

4 

f 

11 

f 

27 

f 

37 

6 

9 

7 

8 

10 

2f 

10 

43 

10 

54 

7 

14 a. m. 

8 

f 

9 

40 

9 

JO 

10 

17 

10 

53 

1 

29 p. m. 

2 

19 

2 

4<5 — 

4 

5 ° 

<5 

52 - 

8 

0 

10 

if 


decl. ac. 
o / 

6 

7 

l 

$> 

8 

8 


8 

7 

7 

7 

8 

7 

6 

f 

6 

7 

7 

7 

7 

7 

6 

7 

5 

6 
6 


48 

if 

2f 

ff 

ff 

7 

38 

10 
37 

ff 

22 

10 

29 

1 

29 

f 4 

53 

22 

o 

f 

11 

19 

29 

16 

2 

58 

ff 

5 ° 


Variavit itaque acus intra diem unum plus quinque 
integris gradibus. 

Die 20 Aprilis, cum toto die vehementer plueret, 
acus tamea turbata fuit continue, maxims variation 



[- 3*3 

nes erant duoram graduum. Non conquievit acue 
ante meridiem diei fequentis. 

Sed te jam nimis diu detinui, vir xftumatiffime } 
ideoque heic fubfiftens me tuae amicitiae tuoque favori 
etiam atque etiam commendo. Vale. 

Stockholm! - ®, calendis Maii, 

1750. 


XVIII. An ExtraSl of a Letter , dated May 
2,17 50, from Mr. Freeman at Naples, 
to the right honourable the Lady Mary Capel, 
relating to the Ruins of Herculaneum. 


Read Feb. 28. "\T OU remember, that, about 7 or 8 year* 

17 S °’ JL ago, the difcovery of Herculaneum 
was greatly talk’d of, and reported to have been fwal- 
low’d up by a violent eruption of Mount Vefuvius ; 
which, by the moft accurate accounts, was in the 
firft year of the reign of the emperor Titus, and 79 
years after Chrift. 

The htuation of this antient city is, as it were, at 
the foot of Vefuvius near the fea, and juft at one end 
of the village of Portici, the palace of the king of 
Naples’s fummer refidence j and, I dare fay, a great 
part of the city is under the faid village. 

I was firft conducted down a narrow paftage, 
which they have dug wide enough barely for two 
perfons to pafs by each other ; and defeended, by a 
gradual Hope, to the depth of about 6 y feet perpen¬ 
dicular. Here I faw a great part of the ancient thea¬ 
tre, being a building in the form of an horlefhoe. 
That part of it, where is fuppofed to be the oreheftra 

R 2 and 



