538 PHILOSOPHICAL TRANSACTIONS. [aNNO 1721. 



than usual, that evening, between 7 and 8 o'clock, I filled a tube with very 

 clean quicksilver, and found the height a little to exceed 30.7-t^ inches. By 8 

 the next morning, a wheel-barometer, which hung in the same room, had risen 

 ^ of an inch higher than it was the night before, when the experiment was 

 made; at 10 o'clock 4- of an inch more; at which time it was at the highest, 

 being a little above 30.8-1- inches ; for about 1 2 at noon it was sensibly lower, 

 and continued falling all the rest of the day. 



When the lower end of the tube was first immersed in the cistern, the quick- 

 silver for some time adhered to the crown of the glass; but upon shaking it 

 fell to the height above-mentioned. 



A Caution to be used in examining the Specific Gravity of Solids, by weighing 

 them in TVater. By James Jurin, M.D. R.S,Secr. N° 369, p. 223. 



That the experiments for finding the specific gravity of solid bodies, should 

 be made with great exactness, if we would so far depend on them, as to draw 

 any inferences from them in natural philosophy, a caution may be useful, which 

 has been but little regarded: viz. that when a dry porous body is to be weighed 

 in water, to discover its specific gravity, it is necessary to extricate the air out 

 of all the small pores and cavities within it, that the water may have free liberty 

 to pervade them. Unless this care be taken, it must needs happen, that the 

 air, which possesses those small cavities, and keeps the water out, will render 

 the solid of less weight in the water, and consequently of less apparent specific 

 gravity than it really is. 



The best way of avoiding this inconvenience, is to set the vessel of water in 

 which the solid body is im merged, under the receiver of an air-pump, and to 

 extract the air out of the body by that means; which will be more easily and 

 exactly done, if the water be first heated over the fire. And where the conve- 

 nience of an air-pump cannot be had, the same thing may be done almost as 

 well, by letting the solid body continue some time in boiling water over the 

 fire. But no solid body must ever be put into hot water, that will in any mea- 

 sure dissolve, or give a tincture to it. 



One instance of the neglect of this caution may be seen in the accounts we 

 have of the specific gravity of the stones taken out of human bladders, which 

 have been commonly found to be but about one half, and some of them have 

 been no more than ^ part heavier than an equal bulk of water. From this 



the trae length of the simple pendulum } on which he continued to noake experiments till almost 

 the time of his death, which happened in 1751, at 76 years of age. He was interred in West- 

 noinster Abbey, in the same grave as his kind master Mr. Tompion. 



