VOL. XLVIII."! PHILOSOPHICAL TRANSACTIONS. 483 



taining that degree. That this may be truly tlie case, the water should be tre- 

 quently stirred, that there may be no difterence of heat in the different parts of 

 the water: this being tlone, the height of the quicksilver appearing stationary, 

 the contact with the screw of the micrometer also remaining the same, for a 

 space of time, it is to be supposed, that the heat of the 3 bodies will be the same 

 as the heat of the water, however different they may be in specific gravit}', &c. 

 The whole difficulty is now reduced to this problem, viz. 



To find the absolute expansion of the basis between any two given degrees of 

 heat, not greater than that of boiling water. 



For this pui-pose, let there be prepared a bar of straight-grained white deal, 

 or cedar, which, it is well known, are much less expansible by heat than any 

 metal hitherto discovered: let the bar be adapted to the instrument in like manner 

 as the other bars intended to be measured; but that the softness of the wood 

 may not hinder the justness of its bearings, let its ends be guarded with a bit of 

 brass let into the wood at the points of contact: to prevent, as much as may be, 

 the moisture or steam of the water from affecting the wood; let it first be well 

 varnished, and then, being wrapped round with coarse fiax from end to end, this 

 will in a great measure imbibe the vapour, before it arrives at the wood. Let 

 the cistern also be so contrived, that the instrument being supported at a proper 

 height in it, the bar to be measured may on occasion be above the cover, while 

 the basis remains in the water: thus will the cover also be a defence against the 

 moisture. Let the water in the cistern be now brought to its lower degree of 

 heat, suppose at or near the freezing point, the basis having continued long 

 enough in the water to receive the same degree of heat, and the wooden bar 

 having been previously kept in an adjacent room, not subject to sudden altera- 

 tions of temperature by fire, or other causes; let the bar be applied to the in- 

 strument, and the degrees of the micrometer and the thermometer read off, and 

 set down : let the wooden bar be then restored to its former place, till the water 

 is heated to the greater degree intended, suppose at or near that of boiling water; 

 the lid being now shut down, and the chinks stopped with coarse fiax, to pre- 

 vent the issuing of the steam as much as possible, let the wooden bar be again 

 brought forth, applied to the instrument, and the degrees of the micrometer and 

 thermometer read off, as before: the difference of degrees of the micrometer, 

 corresponding to the difference of degrees of the thermometer, will express the 

 expansion of the basis between those degrees of heat ; that is, on the supposition 

 that the wooden bar was of the same length, at the time of taking the second 

 measure, as at the first; indeed a measure can hardly be taken without any loss of 

 time, as the whole of the instrument, when the hot measure is to be taken, 

 is considerably hotter than the wooden bar; and, in case of boiling water, the 



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