248 
periodical revolution round a common 
centre. The fame theory he applies to 
triple, quadruple, and multiple fyftems of. 
fiars ; and purfues his conjectures ftill far- 
ther to clufters and groups of ftars, as 
well as to the nebulz, fome of which he 
thinks may be fo diftant, as for the rays 
of light to take two millions of years in 
travelling from them to our fyftem. This 
fubjeét we fhall refume in a future num- 
ber. 
The Rev. Mr. HELLINs communicated 
a Paper on the Rectification of the Conic 
Seétions, in feveral theorems, by means of 
infinite feries ; each cafe was illuftrated by 
a variety of examples. 
At the fame time were read, Obfervations 
on heat, and the aétion of bodies which in- 
tercept it, by M. Prevot, Proteflor of 
Natural Philofophy at Geneva. This pa- 
per confilts chiefly of inferences from Dr, 
Herfchell’s experipients on the tran{miffion 
of heat by different refraéting mediums, 
LINNEAN SOCIETY. 
The late Mr. WiLLiam Curtis has 
given, in theTranfaétions of this Society, a 
very curious paper, containing obferva- 
tions on Aphides, with a view of fhewing 
that they are the principal caufe of blights 
in plants, and the fole caufe of the boney- 
dew. 
The Aphis, or dblighter, fays this Au- 
thor, forms a highly interefting tribe of in- 
fe&ts. In point of number, the individu- 
als of the feveral {pecies compofing it, fur- 
pafs thofe of any other genus in this coun- 
try. ‘hey live entirely on vegetables, 
prefer young fhoots, and often infinuate 
themfelves into the very heart of the plant 
before they are difcovered. But, for the 
moft part they befet the. foliage, and are 
always found on the under fide of the leaf, 
as being mot tender, and as it affords 
them protection from the inclemency of 
the weather, and other injuries to which 
they would be otherwife expofed : fome- 
times, however, they befet the root, and 
deftroy the plant ; but fuch inftances are 
Tare. 
Thefe infeéts are more abundant in fome 
years than others. In 1793 and 1798 they 
deftroyed the hops. In 1794. the hops were 
free from them, but the peas and beans 
fuffered very ferious depredations. In 
1798 the beans were almoft univerfally 
cut off by them. They are often extreme- 
ly injurious to plants in hot-houfes, green- 
houfes, and under frames. 
Locufts and caterpiliars are furnifhedwith 
firong jaws: the Aphis is provided with a 
hollow pointed trunk, with whichit pierce s 
2 
Proceedings of Learned Societies. 
[ O&ober a 
the plant, and imbibes the juices; hence it, 
becomes fickly and exhaufted, being, in 
fact, bled to death. The Aphis, (fays 
Mr. Curtis} comes into the world wita. 
its feet foremoft, and this not at once, but 
time is yiven to the young Aphis to learn 
the ufe of its legs before it is entirely dif- 
engaged fram the mother. On the autho- 
rity of M. Bonnet, we are told, that in 
this tribe of infeéts a fingle impregnation 
is fufficient to produce many fucce‘live ge- 
netations : and, according to M. Reau- 
mur, in five generations the produce of a 
fingle Aphis would be more than five 
thoufand nine hundred and four million of 
individuals. At certain feafons both males 
and females in moft {pecies of Aphides ac- 
quire wings. The very excrement of thefe 
curious infects merits the attention of the 
naturalift : itis of a highly faceharine na-. 
ture, and is, in fact, that fhining clammy 
fubftance well known -under the name of 
the boney-dew. Mr. Curtis goes on to 
fhew that this fubfiance cannot fall from 
the atmofphere, as fome authors have 
thought ; nor iflue from the plant, accord- 
ing to others : for, if it fell from the at- 
mofphere, it would cover every thing ; 
whereas it is never found but on living 
plants : if it exuded from the plant, it 
would appear on all the leaves generally, 
and uniformly ;-inftead of which its ap- 
pearance is extremely irregular. 
It appears that feafons remarkable for 
the deftruétive influence of thefe infeéts oc- 
cur on the average once in four or fix years; 
that they are increafed by mild winters, 
and deftroyed by fevere ones’: that they 
will bear immerfion for a confiderable ° 
length of time without any apparent inju- 
ry ; and that nothing but {moke, in a 
confined place, has been found as a cer- 
tain and effectual remedy again them.— 
It is worthy of notice, that this tribe of, 
infects is food to all the different {pecies of 
Coccinella, or common Lady-bird : toihele, 
therefore, we are indebted for the diminu- 
tion of the number of Aphides. The 
Ichneumon fly is another formidable ene= 
my tothe Aphis ; and fo is the earwig, 
efpecially to fuch as refide in the curled-: 
up leaves of fruit-trees. Many foft-billed: 
birds alfo generally feed on thefe infects. 
Mr. Curtis controverts the opinion that, 
blights are brought by eafterly winds, or. 
that they take -poffeffion only of fickly 
plants; the Aphides do not. {warm like 
bees or ants, and emigrate in large bodies, 
but each pair fly_off together without 
waiting for any other. Detaching itfelf 
from the plant, each; purfues a different 
route, intent on the great bufinefs of mul- 
tiplying 
