208 On the Construction and use of Portahie Barometers. [Oct. 



attached to the left side of the instrument by three hinges, and closes 

 by 3 hooks and a small lock and key. Hollows cut out in the cover 

 admit the heads of the adjusting screws, and in it are also imbedded a 

 small delicate thermometer, a hook with a conical screw, to screw into 

 a tree or post, to hang the instrument on, and an ivory plate with the 

 aqueous tension in inches engraved on it from 32^ to 120° of Fahren- 

 heits. A brass swivel ring attached to the top screw serves to hang the 

 instrument up by. 



13. To take an observation the tube is filled as directed above, and 

 the case being held by an assistant to an angle of 45° with the lower 

 end over a plate, or handkerchief on the ground, the cistern is filled with 

 mercury until it runs over. In this the end of the tube is immersed 

 while closed with the finger, and it is then laid in the groove, the secur- 

 ing piece of brass turned down, and the instrument held vertical, the 

 surplus mercury in the cistern running over into the plate or handker- 

 chief. The bar is then screwed up as far as it will move, and some of 

 the mercury in the cistern, is allowed to run out, by a little channel 

 through the wood, which is stopped by an ivory pin ; until the end of 

 the ivory pin attached to the bar becomes visible (which projects about 

 six-tenth inches below the edge of the glass plate). The point is then 

 brought into exact contact with the mercury in the cistern by the ad- 

 justing screw, and the height read off* by the vernier. As tubes | an 

 inch diameter in the upper part, can be used in this manner, the effect of 

 capillary attraction is avoided, and the length of the mercurial column, 

 added to the aqueous tension for the temperature, gives the correct 

 pressure, with a degree of precision never exceeded in portable barome- 

 ters, and comparable even with the best standard instruments. 



14. In publishing barometrical observations, whether for altitudes or 

 for a series of meteorological observations, observers should be always 

 careful to describe their instruments, and to give certain evidence of their 

 being in good order. For want of this many series of observations re- 

 corded, are hardly worth the trouble of making them ; and it must be 

 recollected, that having compared a barometer at one time with a stand- 

 ard instrument, is no proof of its being in good order at another. It 

 might seem to some that the last remark was somewhat gratuitous, 

 but in turning over some recorded meteorological observations, I 

 find a gentleman remarking that his barometer " had been compared 

 with Mr Prinsep's standards, and had proved correct, and that it had 

 since been emptied, and the mercury purified'' —vfhich is almost as 

 much as to say that his observations were worth nothing, although Sir 

 John Herschel in remarking on them was too polite to say so. In general 



