472 Professor STEVELLY on a new 
taining the height at which it had ranged at any instant of past time that our re- 
searches may render desirable. 
But farther, the same sheet of paper being used on many successive days, for a 
great length of time, it is obyious that the points at which the point of the pencil 
arrived oftenest would become darker than those points at which it was only occa- 
sionally found at the same hour, and therefore the succession of these darker points 
would at length trace out the true curve of mean diurnal oscillation. In this case, 
and perhaps indeed always, it would be best to have the sheet of paper strained upon 
the surface of a cylinder, which the clock-work should cause to turn round once in 
twenty-four hours. In this way it is obvious that nearly all the labour and tedium of 
observing, recording, and afterwards reducing actual observations to mean results, 
may be avoided. 
Most, if not all of these advantages, I conceive, may be attained by the use of an 
instrument, one modification of which I had the honour of hastily describing to the 
subsection of useful arts at the late meeting of the British Association in Dublin. 
I shall now give a more full and a more general description of the instrument, 
together with the formule, which will in all ordinary circumstances of variety of 
construction, give the connection of its scale with the scale of the common 
barometer. I shall also briefly point out the manner in which its indications are af- 
fected by changes of temperature, at least so far as may be required for making the ne- 
cessary corrections. 
During the oscillations of the common barometer, when it falls, a certain quantity 
of mercury is added to that already in the cistern, which of course increases its weight 
by so much. On the contrary, when the barometer rises, mercury retires from the 
cistern into the tube ; the cistern thus becoming by so much lighter than it was before. 
If then the tube of the barometer be firmly sustained in its place, but the cistern be 
suspended by any mechanical means, so as to descend by arithmetical distances for 
equal additions to its weight, and to rise similarly when its weight has been similarly 
diminished ; an index carried by the cistern may be made either to point to a fixed 
scale placed beside the instrument, as in the common barometer, or to mark on a 
sheet of paper a variety of positions corresponding with the synchronous variations 
of the height of the barometer, while, as will appear just now, the range of the scale 
of this new barometer may be made to bear any proportion, that may be desired, to 
the three-inch scale of the common barometer. 
Although various mechanical means of suspending the cistern may be readily de- 
vised, either on the principle of the lever, or by certain curved surfaces made to turn 
freely on an axis, or even by simple counterpoise, the alteration of the buoyant force 
of a metallic cylinder as it is drawn more or less out of a fluid being made the means 
of restoring the equilibrium, yet for reasons which J shall not now stop to detail, I 
should prefer to any of these, the method of suspending it, derived from the buoyant 
