new Cistern for Barometers. S39 



E, which is tightened by the screwed ring H ; so that by turn- 

 ing the screw F we can withdraw or advance the phmger, and 

 cause the mercury to rise and fall in the tube. When the ba- 

 rometer is used, it is adjusted until the surface of the mercury 

 cuts off the light at the opening G in the siphon tube. The size 

 of the plunger is made equal to the quantity of mercury which 

 will descend into the cistern by the greatest fall of the baro- 

 meter, by which means the cistern may be made of the ex- 

 act size required. For example, if the tube be .2 of an inch in 

 diameter, and the required range from 3^ to 10 inches, that 

 is over a space of 22 inches, the quantity of mercury contain- 

 ed in such a tube is = .691152 parts of a cubic inch, which 

 quantity the plunger is made equal to. 



The advantages I conceive to attend this construction of 

 cistern are, that it is not affected by moisture and heat in warm 

 climates. The adjustment of the surface of the mercury is more 

 easily made, and not liable to change from any motion of the 

 instrument during the time of observation, the mercury form- 

 ing with the cistern a compact mass. From the same cause it 

 is not so liable to be broken in carriage from being carelessly 

 turned up. With leather cisterns, any slight motion during 

 the time of observation may cause some lirk (fold) in the lea- 

 ther to distend itself, and thereby require the adjustments to 

 be gone over,— a circumstance I have experienced in using an 

 instrument of that construction. 



The only other construction of cast iron cistern that has 

 come under my notice, is that forming a cylindrical box having 

 its upper end of wood, into which the tube is fixed. This I con- 

 ceive to have many objections, from the number of corrections 

 required ; first, the difference of capacity between the tube 

 and cistern ; second, for the capillary action ; and third, a neu- 

 tral point must be determined, so that any portion of mer- 

 cury being lost in carriage or otherwise, the series of observa- 

 tion made under such circumstances, with such an instrument, 

 become of no value; whereas with a siphon barometer the 

 quantity of mercury contained in the cistern is in no way con- 

 nected with the results given by that instrument, and besides, 

 the corrections for capillary action and capacity are not requir- 

 ed. Another great objection to the iron box construction of cis-» 



