470 Magnetic Field in the Jefferson Physical Laboratory. 



tides were separated in the same manner from the cement, as 

 well as from the clay of the brick-yard where the bricks were 

 made. 



Far more striking results were obtained on examining the 

 soap-stone sink which is connected with the disturbance in 

 room 13, and which shows upon the upper surface and edges 

 of its slabs free south magnetism of sufficient amount to deflect 

 a compass needle several degrees when brought within an inch 

 of its mass. 



Sixteen grams of dust scraped with glass from the under 

 side of the base gave 1 gram of dark strongly magnetic particles 

 consisting mainly of magnetite in octahedra up to l-2 mm in 

 diameter, which were disseminated through the mass with 

 considerable uniformity. Pencils cut from a similar slab and 

 hung by a thread were strongly attracted by a magnet, though 

 they received no permanent polarity. A chip of the same 

 weighing over one third of a gram was lifted bodily by a one 

 pound magnet with poles approaching each other within -§- of 

 an inch. 



I am not aware that any observation has been previously 

 made of so great a disturbance of the magnetic field from 

 brick-work. In the magnetic observatory at Wiirzburg the 

 brick for which was selected with great care, a variation of 

 •00025 H has been noted.* Lamontf remarks upon a differ- 

 ence of 2' 18" in declination and 0'00014 in the value of H 

 between observations made in the open field near the observa- 

 tory (Munich) and upon an " aus Backsteinen und hydraulischen 

 Kalk, ausserdem Starke Messingtheile enthaltenden Pfeiler" 

 in the building itself. This discrepancy he does not appear to 

 have investigated further. 



It is possible that the Cambridge brick, of which the Jeffer- 

 son Laboratory is built, contains an unusual amount of magnetic 

 oxide, but it is a fact not to be slighted by those engaged in 

 magnetic work that bricks, without special examination, cannot 

 be assumed to have an insignificant magnetic effect. 



In fact, I incline to the belief that in general it is safer to 

 make exclusive use of wood for buildings and piers intended 

 for refined magnetic measurements. 



Jefferson Physical Laboratory. 



* F. Kohlrausch, Wied. Ann., vol. xix. p. 142, 1883. 



f Abhandl. d. k. bayr. Akad. d. Wiss. ; Math.-Phys. Kl., v, p. 24, 1847. 



