VOL. XLV.] PHILOSOPHICAL TRANSACTIONS. 553 



weight of the whole atmosphere: which in our climate is not less than J-o of the 

 whole weight, allowing the barometer to vary from 28 to 31 inches. Again, by 

 an experiment of the late Mr. Hauksbee's in his Phys. Mechan. exp. p. 170, the 

 density of the air varies -f part between the greatest degree of heat in summer, 

 and that of cold in the winter season. So that the air, in a hard frost, when the 

 mercury stands at 81 inches, is near a 5th part specifically heavier than it is in a 

 hot day, when the mercury stands at 28 inches. 



TABLE XI. — From Mons. Homberg and John Caspar Eisenschmid, of tht Proportion of the Specific 

 freights of certain Fluids in the Winter, to the Wtfights of the same in the Summer Season. 



Mercurius 1. 00479 Acetum I.OI6OO 



Aqua pluvialis l.OOSOy Ol. vitrioli 1.02131 



Aqua fluviatilis 1.0081 1 Ol. terebinth 1.02141 



Aqua distillata 1.00815 Aquafortis 1.02637 



Spirit, vitriol 1 .01272 Ol. tartari 1.03013 



Lac bubulum 1.01316 Spir. vini 1.03125 



Aqua marina 1.01351 Spir. nitri 1.04386 



Spir. salis 1 .01467 



According to this table, the increase of the specific weight of common water 

 in the winter above its weight in the summer, is not more than about the i 24th 

 part of the whole; which is little more than half of what Professor Muschen- 

 broek has elsewhere accounted the same; but sure this difference is much too 

 great. Though all fluids are condensed by cold, it is only till such time as they 

 are ready to freeze: for on the freezing they immediately expand again, so as for 

 the ice to be specifically lighter than the fluid of which it is formed, and to swim 

 in it ; Muschenbroek gives the specific weight of ice to be to that of water com- 

 monly as 8 to 9. 



Of the Experiments made by some Gentlemen of the Royal Society, to Measure 

 the Absolute Velocity of Electricity. By Mr. W. Watson, F. R. S. 

 N''489, p. 491. 



Mr. W. laid before the R. S. and printed in N° 485, an account of what had 

 been done by some gentlemen, in order to ascertain the respective velocities of 

 electricity and sound; from which it appeared, that through a space measuring 

 6732 feet, the electricity was perceptible in a quantity of time less than -"oVo of 

 a second. But the gentlemen concerned were desirous, if possible, of ascer- 

 taining the absolute velocity of electricity at a certain distance; and a method 

 had been thought of, by which this might be determined with great exactness. 



Accordingly, Aug. 5, 1748, there met at Shooter's Hill for this purpose, the 

 president of the r. s. the Rev. Mr. Birch, the Rev. Dr. Bradley, astron. royal; 

 James Burrow, Esq., Mr. Ellicot. Mr. George Graham, Rich. Graham, Esq. 

 the Rev. Mr. Lawrie, Charles Stanhope, Esq. and Mr. W. who were of the 



VOL. IX. 4 B 



