134 PHILOSOPHICAX TRANSACTIONS. [aNNO 1751. 



made to lift 28 Troy ounces, and sometimes more. And when these bars are 

 thus impregnated, they will give to a hard bar of the same size, its full virtue ia 

 less than 2 minutes : and therefore will answer all the purposes of magnetism in 

 navigation and experimental philosophy, much better than the loadstone, which 

 is well known not to have sufficient power to impregnate hard_ bars. The half 

 dozen being put into a case (fig. 7) in such a manner, as that 2 poles of the 

 same denomination nmy not be together, and their irons with them as one bar, 

 they will retain the virtue they have received . but if their power should, by 

 making experiments, be ever so far impaired, it may be restored without any 

 foreign assistance in a few minutes. And if, out of curiosity, a much larger set 

 of bars should be required, these will communicate to them a sufficient power to 

 proceed with, and they may in a short time, by the same method, be brought 

 to their full strength. 



f^lf. An Aurora Borealis observed at the Hague, Feb. 27, N. S. 1750. By 

 Peter Gabre, J. K. D. Phys. Astron. et Math. From the Latin, p. 3Q. 



This very luminous aurora borealis was in the form of an iris, the extremities 

 of which extended from the eastern horizon to the west, and its top towards the 

 south near the zenith, rising near 80° above the horizon. Its breadth at the 

 vertex was about 2°, but narrowed to cusps at the two extremities. The 

 middle of the arc emitted a strong white light ; but weaker towards the sides. 



Fill. Further Observations on the Cancer Major. By Mr. Peter CoUitison, 



F.R.S.y p. 40. 



That the cancer major, and all species of crabs, cast their shells, is certain ; 

 but at what season of the year, or how frequently, is not exactly to be deter- 

 mined ; but it is believed to be annually at the beginning of the summer, sooner 

 or later, according to the greater or less strength of the crab. 



There is in the under part of the shell a suture in the form of a crescent, 

 which retains a part of the shell of the same figure. At the time of casting the 

 old shell, this suture opens, and leaves a space sufficient for drawing out the 

 whole body ; after which the thorax drops its breast-plate, and then the legs 

 quit their crustaceous coverings. The carcase now is left inveloped with a soft 

 skin like wet parchment. In this helpless state the crab is incapable of moving, 

 but it lies at the bottom of the sea, between the rocks, till its new shell ac- 

 quires a sufficient hardness and consistence, fit for its defence, and its limbs 

 grow strong enough to bear its weight, and carry it about, to perform its ne- 



the pan with the leather, and places it on a gentle flat fire, covering and surrounding it with char- 

 coal. The pan being brought to somewhat more than a red heat, he keeps it so about half an hour, 

 and then suddenly quenches the bars in a large quantity of cold water. — Orig. 



