76 Meteorological Summary for 1 822. — Essex. 



P. S. — I have no wish to occupy your pages to the exclusion 

 of your more scientific communications, but must beg to ob- 

 serve, that I believe, if correct observations of the barometer, 

 the attached and external thermometers, were made in dif- 

 ferent parts of the counti-y for one year, or even for a much less 

 period, and a mean of these observations taken as above, and 

 compared, we might, by that useful instrument the barometer, 

 be enabled to ascertain the exact elevation of most parts of the 

 countr}'. It must be evident to every one, that the mean alti- 

 tudes of the barometer obtained from a great many observa- 

 tions would so far reduce the errors arising from imequal at- 

 mospheric pressure, as not in the least to affect the results 

 founded on such observations ; for though an equilibrium of 

 pressure may never take place over the whole extent of this 

 island, at any given increment of time, yet, nevertheless, any 

 change in one place is generally followed or preceded by a si- 

 milar change in others; and therefore, the means in such case 

 cannot be much affected by this circumstance. But there is 

 another source from which errors may arise, and which, if not 

 attended to, will in a great measure render such comparative 

 observations useless, — I mean the constant variation of the al- 

 titude of the mercury in the basin. This equation can always be 

 found when barometers expressly constructed for the purpose 

 of measuring altitudes are used ; but in our common portable 

 chamber barometers there is no contrivance for that purpose. 

 To render the common barometers useful for measuring alti- 

 tudes, the exact point of zero, with the ratio of the area of an 

 horizontal section of the mercury above the orifice of the tube 

 in the basin, to that of the column itself, should be engraved 

 on the plate of the instrument ; whence, by this simple plan, 

 the most common observer will be able to find this equation, 

 and which, being applied to the altitude shown by the vernier, 

 will always give the exact height or length of the mercurial 

 column above the surface of the quicksilver in the basin ; ob- 

 serving to add the quantity thus found when the barometer is 

 above zero, and subtract the same when it stands below the 

 said point. The simple method here pointed out would, if 

 adopted, completely establish the universality of this interest- 

 hig and useful instrument. T. S, 



In page 44'4 of your last Number, your correspondent Mr. 

 Innes is correct in saying die ]) makes the first unpression on 

 the O's limb to the left liand. I know that will be the case at 

 Greenwich, and I dare say the like will take place at Aber- 

 deen. 



