262 



On the Construction and use of Portable Barometers. 



[Oct. 



the screw being replaced, the instrument is finished. The bottom part 

 is then turned to " not portable," and the instrument is set upright. In 

 this position, the air which was in the movable bottom of course passes 

 up through the two apertures, and the mercury descends into the space 

 the air had occupit d, thus leaving a part of the upper cistern empty 

 to allow of the descent of the mercury in the tube. The instrument is 

 then compared with a standard barometer, and the scale adjusted so as 

 to shew the proper height. The heiglit of the column indicated at this 

 time is called the " neutral point." When the instrument is prepared 

 for transport, it is first inverted, by which the mercury again fills the 

 vacuum in the top of the tube, and also fills completely the upper part 

 of the cistern, and the bottom being turned to " portable" all motion 

 of the mercury in the tube is prevented. But the consequence is, that 

 no allowance is made for the expansion of the mercury in the tube, 

 and supposing the instrument packed in London at the temperature of 

 32°, and that on the voyage it was exposed to a temperature of 140 

 (which is quite probable), the expansion of the mercury in the tube 32 

 inches long would amount to more than three-tenths of an inch, and the 

 wooden cover of the cistern being nearly 50 times larger than the tube, 

 this expansion of the mercury would act with an irresistible force, mul- 

 tiplied 50 times, to force out the cover, or the mercury into its pores. 

 There can be little doubt that the cover, after being exposed to this 

 enormous pressure,would become loose, after which the expansion of the 

 small quantity of air which would get into the cistern after the instru- 

 ment was exposed to cold again, would be able to force out more mer- 

 cury from the cistern, an effect which would go on increasing until the 

 instrument was quite destroyed : or if set upriglit with the movable 

 bottom part turned to " portable," the weight of the mercury in the top 

 of the tube,would exert a pressure increasing as the barometrical pressure 

 at the place decreased; and most likely quite enough to force out more 

 mercury from the loosened joints. Another defect in the instrument 

 arises from the cover of the cistern being made of wood. In the instru- 

 ment examined, this has shrunk so much, that it is quite loose, so as to 

 afford a very easy escape to the mercury. This part should have been 

 made also of iron, with a small plug of loose grained wood inserted in it. 

 Another imperfection lays in the scale not being connected with the 

 cistern by a brass rod, which it should have been, by which the uncer- 

 tain longitudinal expansion of the wood mounting would have been coun- 

 teracted. A very strong corroboration of the remarks of the late Lieut. 

 Braddock on this last imperfection may be seen in the idstrument ex- 

 amined. The scale is H inches in length, and is attached at each end 



