EXHAUSTING MACHINl^ BY MERCURY. 163 



communication with the atmosphere was cut off. The cock 

 was then opened at the bottom of the tube E, the mercury 

 descended, and some sort of vacuum was produced. 



In order to see the state of this vacuum, as we had an State of the 

 iron ttibe, I suggested a gauge, which was easily intro- ^l^h'a^a^ee. 

 duced before filling, and floated on the surface of the mer- 

 cury within the tube. The index of this gauge, passing 

 into the vacuum, exhibited the height of the mercury. But 

 I must confess, though every part was perfectly tight, and 

 I believe well made, the mercury well dried, and the ma- 

 chine much agitated in filling, to dislodge the air, it did 

 not afford the satisfaction sought after. From this failure 

 I am inclined to think, that so good a vacuum cannot be ^"^ equal io 

 obtained by such a mercurial apparatus, as with a pump of ^ir pump.^ 

 the best construction, that will indicate an exhaustion to 

 the -^-^ of an inch with the nicest test, a siphon gauge. 



Should you think these observations worthy a place in 

 your Journal, they are at your service. 

 1 am. 

 Your very humble servant, 



Sept. 9, 1808. R. BANCKS. 



P. S. Could Dr. Traillfs instrument be made perfect, 

 and to supersede the use of our best pumps in nice experi- 

 ments, many of those that may be deemed of importance 

 must be laid aside, from the expense that would be incur- Expensive, 

 red, as the apparatus could be made only in iron or steel, 

 and would come much more expensive than it does at pre- 

 sent. 



ANNOTATION. j 



IN giving Dr. Traill's instrument, I never had an idea, T^e vacuum I 

 that a perfect vacuum would be obtained by it. The great "° ^^^ **^ * 

 difficulty of freeing the mercury from air, it is probable, must 1 



ever prove an insuperable obstacle to our complete success. 

 I think farther it is very questionable, whether the contact j 



between the mercury and the sides of the tube would be so 

 complete, as perfectly to prevent any air from insinuating 

 itself between theni. But the grand utility of the machine ' 



M 2 invented \ 



