TRANSACTIONS OF THE SECTIONS. 417 



the atmospheric pressure. The iron stopcock and cap may 

 now be removed, by cautious application of first a warm and 

 then a hot iron rod to the cement. 



The mercury intended for the purpose of the barometer 

 should be first distilled, and then well agitated, about an ounce 

 or less at a time, in phials capable of holding one ounce and a 

 half. Previously to introducing the mercury into the tube it 

 should be well boiled in a crucible, of porcelain or Wedgwood 

 ware, and should be used just before getting cold, at a tempe- 

 rature of 90° or 100°, the tube of the barometer being also 

 made a little warm by cai'cful exposure to a charcoal fire. 



The process now described is believed by the author to be, 

 when carefully performed, in every respect equal to that of 

 boiling. The wheel-barometer made in this way has been com- 

 pared with other instruments with boiled tubes and of un- 

 doubted excellence, amongst others with a fine mountain baro- 

 meter on Gay-Lussac and Renard's principle, which had been 

 compared with the standard of the Royal Society in Somer^set 

 House, and with that in the observatory at Paris. The dif- 

 ferences from this instrument, when both were placed in the 

 same room, were very minute. It is found to be more sensible 

 than a very finely boiled tube, carefully prepared by that emi- 

 nent maker Mr. Cox, of Plymouth, with a scale and vernier 



divided to s^th of an inch, set up in an adjoining room. 



On a new Method of Constructing a Portable Barometer. 

 By John New^man, Mathematical Instrument Maker. 



The object of this construction is to make barometers port- 

 able without the use of a leather bag, which has always been 

 a defective part of the instrument. 



The method adopted is to have a cylindrical cistern of iron 

 in two parts, rather longer than usual, the upper part, or 

 chamber, or that to which the cap is fastened, which connects 

 to the tube, being about three times the length of the lower 

 part, of the same diameter, moving round upon a pin, and 

 secured by a screw and collar. The two chambers thus formed 

 communicate internally in one situation by means of holes in 

 the divisions, through which the mercury flows upon invert- 

 ing the instrument. The vacant space, or that intended to 

 receive the mercury from the tube when the barometer falls, is, 

 when the instrument is in use, in the upper part of the upper 

 cistern, the lower one being full. Upon inverting the instru- 

 ment, the mercury flows from the latter into the formei-, which 



1833. 2 E 



