On the Insulated or Safety Compass. 365 
Exp. 9.—This fluid being completely freed from muriate of 
soda, and highly concentrated, was decomposed with subcarbo- 
nate of potash. It-yielded a copious white precipitate, which, 
after being thoroughly ignited, gave 1:25 grain of sulphate of 
magnesia (taking 136°68 of magnesia to be equal to 100 of sul- 
phate of magnesia) to be present in one gallon of the water. 
Exp.10.—The dilute fluid obtained in process 3, yielded a pre- 
cipitate by oxalate of ammonia and nitrate of barytes ; and being 
on a further examination found to contain nothing but sulphate 
of lime, it was decomposed by barytic water; and taking 100 
parts of sulphate of barytes to be produced by 71 of sulphate 
of lime, gave three grains of sulphate of lime to one gallon of 
water. 
The composition of the Thetford chalybeate water is therefore 
as follows : 
Contents in one Gallon. 
Carbonate of iron... ..  .. oe §=62°7 5 grains. 
Muriate of magnesia» .. .. .. 3°25 
Misridte:ofslimes : si. 9351) abe eeS 12225 
Sulphate of magnesia... 9.2. «. 1°25 
Muriate of-soda., .. es (es «2 2°125 
sulphate: of; limes: ef. 0) aie ten Oring 
14-625 
Carbonic acid gas .. .. 12:07 cubic inches. 
Oxygen gas MNES! Pe MP ie Ibi | 
Atmospheric air Biagio 
LXII. On the Insulated or Safety Compass lately invented by 
Mr.Jennincs. By James Horspuren, Esq. Hydrographer 
to the Honourable the East India Company. 
To Mr. Tilloch. 
Dear sir,—As the pages of the Philosophical Magazine are 
always open to give publicity to the labours of those who are 
gifted with superior talents, I trust you will be able to afford 
room for a few observations on the insulated or safety compass, 
lately invented by Mr. Jennings, which is not yet sufficiently 
known, although it will probably add greatly to the security of 
navigation and commerce, when brought into general use. 
Several men of science have been convinced, by attending to 
experiments made by Mr. Jennings, and his liberal explanation, 
that he has discovered a method of arresting the progress of the 
magnetic fluid, and securing the magnetic needle from being 
disturbed 
