92 Mr. Bennet's Experiments on a new 
existence of a magnetic atmosphere over the earth, where the 
magnetic fluid being rarefied at one pole, and condensed at the 
other, occasions the polar direction of the needle, of so much 
use in navigation. 
EXPERIMENT XIV. 
At the request of Dr. Darwin, I repeated an experiment of 
Mr. Cavallo, on the increased attraction of iron filings by 
effervescence with diluted vitriolic acid, inserted in the Philo- 
sophical Transactions, Vol. LXXVII. and according to his 
direction, I placed two ounces of iron filings in an earthen 
vessel, near the south end of the needle, and first pouring in 
five parts of water, and then one of good vitriolic acid, a brisk 
effervescence ensued ; but the needle could not be perceived 
to move. This I repeated six times, sometimes supposing it 
moved a little, but was not certain, and despaired of success 
till I recollected of how much importance it was to place the 
filings in a proper direction as to the earth's magnetic atmo- 
sphere; for hitherto, although the filings were placed at right 
angles with the point of the needle, yet the bottom of the ves- 
sel was as much below as the upper surface of the filings was 
above it. I then took a quantity of filings tied up in a paper, 
and presenting them to the needle, I found that the bottom 
always repelled, and the top attracted the north pole ; it was 
therefore necessary to place the bottom of the vessel even with 
the needle, or rather above it, which circumstance Mr. Ca- 
vallo did not mention. I then placed the vessel at such a 
distance, that the needle was drawn one degree from its former 
position; then on adding the w r ater and vitriolic acid as before, 
the needle came about one degree still nearer ; yet it appeared 
