mea/uring Heights with the Barometer. 685 
having acquired the heat of boiling water, it is lengthened 
t<to"6^o parts of an inch : alfo, that the fame column, fuffering 
a condenfation by 3 a° of cold, extending to the zero of 
FAHRENHEIT, is fhortened parts, the weight of the 
atmofphere remaining in both cafes unaltered; but that, 
in the application of the barometer to the meafurement of. 
altitudes, fince the preffure and length of the column 
change with every alteration of vertical height, the 
equation, depending on the difference of temperature of 
the quicklilver, will neceffarily augment or diminifh by 
a proportionable part of the whole. Thus, if the weight 
of the atmofphere fhould at any time be fo great as to 
fuftain 3 1 inches of quicklilver, the equation for dif- 
ference of temperature will be juft T ~th part more than 
that for 30 inches; at 25 inches it will be {ths ; at 20 
inches yds; at 15 inches^; and at 10 inches only |d of 
that deduced from experiment. 
It is upon thefe principles that the annexed table of 
equation hath been conftructed, for differences of tem- 
perature extending to 102° of the thermometer, and for- 
heights of the barometer from 1 5 to 3 1 inches ; beyond 
which limits, it is not probable, that many barometrical 
obfervations will be made. The firft or left-hand co- 
lumn, fhews the height of the barometer for every half 
inch, from 31 to 25 ; thence for every inch downwards 
to 
