VOL. XXXV.] PHILOSOPHICAL TRANSACTIONS. 275 



Sept. 9, 17*^ 20™ 30" The 3d satellite fully immersed. 

 10, 10 \Q O The first satellite immersed. 



^ Solar Eclipse observed at the same Place, Sept. 25. 

 At 5 17 22 Beginning of the eclipse. 



The sun's diameter measured exactly 16' O". 



^n Account of a Machine for measuring any Depth in the Sea, with great 

 Expedition and Certainty ; contrived by J. T. Desaguliers, LL. D, and R. S. S. 

 and by the Rev. Mr. Stephen Hales, F. R. S. N° 405, p. 559. 



There have been several machines contrived for measuring the different 

 depths of the sea, especially such as could not be determined by the lead and 

 line ; but as those machines consisted of two bodies, the one specifically lighter, 

 and the other specifically heavier than water, so joined together, that as soon 

 as the heavy one came to the bottom, the lighter should get loose from it, and 

 emerge ; and the depth was to be estimated by the time of the fall of the com- 

 pound body, from the top to the bottom of the water, together with the time 

 of the emersion of the lighter body, reckoned from the disappearing of the 

 machine, till the emergent body was seen again, no certain consequence could 

 be drawn from so precarious and complex an experiment. 



For, even in still water, and in the same place, the time will hardly be the 

 same in two experiments : much less will this machine answer in the sea, on 

 account of waves and currents, and many other causes. 



But as the pressure of fluids in all directions is always the same at the same 

 depth, a gauge which exactly discovers what the pressure is at the bottom of the 

 sea, will show what is the true depth of the sea in that place, whether the 

 time of the descent of the machine be only a minute or two, or 20 times as 

 long. 



The Rev. Mr. Hales, in his Vegetable Statics, describes his gauge for esti- 

 mating the pressures made in opaque vessels, where honey being poured over 

 the surface of mercury in an open vessel, rises on the surface of the mercury as 

 it is pressed up into a tube whose lower orifice is immersed into the honey and 

 mercury, and whose top is hermetically sealed. Now as, by the pressure, the 

 air in the tube is condensed, and the mercury rises, so the mercury comes down 

 again when the pressure is taken off", and would leave no mark of the height to 

 which it had risen ; but the honey, or treacle, which does better, which is on 

 the mercury, sticking to the inside of the tube, leaves a mark, which shows 

 the height to which the mercury had risen, and consequently makes appear 

 what was the greatest pressure. 



NN 2 



