Description of the Mountain Barometer, 61 



ncas for those with a long iscale reading from 20 to 31 



inches. 



I am, sir, your humble servant, 



H. C. Englefield. 



P. S. On comparing several havoiiieters made by Mr. Thomas 

 Jones, since this description was .first written, I find that in 

 some of them ihc mercury does not take its true height on 

 turning up the instrument, quite so quick as in the two 

 which he first constructed for me. This difference is owing; 

 to the greater closeness of fibre in some pieces of box wood 

 than in others, but it does not affect the accuracy of the in- 

 strument. In order to give a quicker action to these baro- 

 meters I advised Mr. Jones to bore a small hole or two in 

 these cisterns, and insert a j)in of open grained wood into 

 them. This answered perfectly well; but a curious circum- 

 stance occurred : when deal or willow wood pins were in- 

 serted, the mercury, when shaken for some time, passed 

 through the pores of these woods in the form of a fine black 

 powder, and it was necessary to substitule ashen pins to 

 confine it in the cistern. It may not be superfluous to say, 

 ihcit the weight of this barom(;ter is less than a pound and 

 a half. The weight of Ramsden's last improved barometer 

 ' is 44" pounds, and that of his earliest about G';- pounds. [ 

 subjoin a few observations by which the accuracy of this 

 barometer may be fairly estimated. 



On R'lclnnond HilL 



iSOfT. Barom. T.herm. Results. 



Jan. 1, Hill top - - 29'540 44 

 Thames* side 2i)-68G 



Feet. 

 - 133 



134 



'i5i - ... 137 



Feb, 



