44 PHILOSOPHICAL TRANSACTIONS. ' [aNNO I672. 



elsewhere teach, that the atmosphere is incumbent as a heavy body upon the 

 terraqueous globe. 



Extract from some Letters of Dr. Johi TVallis to the Editor, 1672, Sept. iQ, 

 &c. concerning the Suspension of Quicksilver ivell purged of Air, much higher 

 tlian tlie ordinary Standard in the Torricellian Experiment. N° gi, 

 p. 5 J 60. 



I am not sorry to find in your Transactions for the last month, that M. 

 Huygens endeavours to account for that odd phenomenon in the Torricellian 

 experiment, of which I give an account in my treatise De Motu, Cap. 14 Schol. 

 prop. J 3. The phenomenon is this : 



Whereas in the Torricellian experiment, the quicksilver contained in the in- 

 verted tube, how long soever, whose open orifice C, (fig. 6, pi. l) is immerged 

 in stagnant quicksilver, does usually fall down to the height of about 29 inches 

 above the surface of the stagnant quicksilver AB, and there remains suspended, 

 as at I : if the quicksilver be well cleansed from air, it has been found to stand 

 top-full, much higher, even to the height of 75 inches, and how much higher 

 it may stand, we cannot tell; but upon the admission of the least air, or a con- 

 cussion of the tube, it falls down to the usual standard. 



Two reasons I did there hint, though not perfectly satisfied in either : the 

 one of my own, concerning the spring of the air, necessary to put heavy bodies 

 in motion, not impelled by any other force : the other of my Lord Brouncker, 

 that there might be in the air yet a greater weight or pressure than is necessary 

 for the height of 29 inches, in case there be nothing but the bare weight of 

 quicksilver to be supported. 



I find M. Huygens falls in with that of my Lord Brouncker, save that what 

 we comprehend under the name of air, he calls a more subtile matter : which 

 alters not the case at all, but only the name. M. Huygens here, by air, seems 

 to understand that feculent matter arising from the earth and water's effluvia, 

 which are intermingled with this subtile matter. We mean by air, the aggre- 

 gate of both these, or whatever else makes up that heterogeneous fluid wherein 

 we breathe, commonly called air ; the purer part of which is Mr. Hobbes's air; 

 and the feculent of it is M. Huygens's air. — And therefore, where I speak of 

 vacuity caused by the Torricellian experiment, or such other ways, I do expressly 

 caution not to be understood as affirming absolute vacuity, but at least an ab- 

 sence of that heterogeneous mixture which we call air, such as that is wherein 

 we breathe ; without disputing against the purus aether of Mr. Hobbes, or the 



