10 
GLEANINGS IN SCIENCE. 
and A its mean density. Now these two quantities, 14394 A and 5.020 S . should 
be to each other, in the proportion of 39-1154, — the pendulum at the level of the 
sea. representing gravitation at the surface of the earth, — to -0070, the portion of 
gravitation at the summit of the mountain due to the attraction ol the mountain. 
By the observations of M. deSaussure and other geologists, Mont Cenis is chiefly 
composed of schistus, marble, and gypsum ; the specific gravities of which sub- 
stances were ascertained, from numerous specimens in the possession of M. Carlmi, 
to be respectively as follows : — 
Tile schistus . . . 2 HI. 
The marble ... 2 86. 
The gypsum . . . 2-32. 
In the absence of a precise knowledge of the quantity and position of each of 
these three component parts, we may take the mean, 2.66, of their several densi- 
ties as approximative!}' the density- of the mountain, = S . We have then 
5.02 5X39- 1154 
A = =4.77, 
1439 4 X- 0076 
a result little differing from that of Cavendish, as recently corrected by Dr. Hutton, 
and still loss from that of the Schehallien experiments. 
The most hypothetical element of this calculation, is the width assigned to the 
base of the mountain; but by the very nature of the question, it has hut little in- 
fluence on the final result ; since, by even doubling the assigned diameter, the 
total attraction would hot be altered a twentieth. In regard to the me an density 
of the mountain, if it were taken at 2-75, instead of 2.66, that of the earth would 
result 4.94, instead of 4.77, as given above. 
Ill— ON ARTIFICIAL COBWEBS FOR MICROMETERS. 
By R. C. Gorins, M. D. 
[From the Quarterly Journal of Science and Arts,Vol. I. JV. A'.] 
A common cobweb has always appeared to me to he a very strong substance 
in proportion to its extreme tenuity, for a bulky spider will frequently depend 
from a single fibre of great length ; — small insects are incapable of overcoming the 
resistance of the meshes of a well ; — which will also endure for a considerable time 
in wet and windy weather without requiring much repair. Nevertheless, I have 
always found astronomers extremely nervous about the threads of their microme- 
ters, which, it is said, may be destroyed by the trifling current of air which passes 
through the tube of a telescope when its objective and eye glasses are displaced • 
probably because the cobwebs, being of animal origin, are 'subject to decomposition 
m course of tune, for I do not think a cobweb, fresh from the body of a spider, 
could be so easily affected. Be this as it may. I have accidentally discovered a mode 
. of makmg artificial ones, which are not. subject to decomposition,— which mav lie 
stretched by mechanical violence to double their length without snapping, return- 
mg again to their ongmal dimensions,- which may be blown upon by a powerful 
pair of bellows, with its nose close to them, without injury, and which can be 
qK substi r z the 
it must be left to Messrs. Troughton and Sims, who have certainly irfvedl at^the 
extreme verge of improvement in the construction of micrometers; m determine 
th< j . point at issue between me and the spider. Thp frill T 
making them :_Procttre some of the purejt 
caoutchouc extremely thin, and put it into a. small wide rnornlmd h" 6 ! haves T e 
not to till it more than one quarter ; pour in the turpltW m th P V t ’ ob , servmg 
it carefully with a cork and bladder Let Ibis be worn in G, ’ T ‘T Se ? 
days, anil the Indian rubber will absorb the essential oil and V P ° < * e for ? , . ew 
creased m hulk ; then let a portion of it he put into a^hcr t ^eatly ,n ' 
turpentine and in a lew days it will he completely dissolved Ti 'T? 
there should be more turpentine employed in the firsUnsmor li S necessaI 7 ‘ hat 
reduce the Ind.a rubber to the consistence necessary ,o ?or i ' an , ls required to 
alh,w mipurmes to subside to the bottom of the phial Vi l c'<>! ' -s. m order to 
lution of the caoutchouc ; for if any i, articles of ’ , , to ,usure !he complete so- 
any particles oi it are left untouched, smooth even 
