174 PROCEEDINGS OF THE AMERICAN ACADEMY 



solute measure is some length. The length that enters the equation 

 is the diameter of the torsion wheel. This was determined under a 

 microscope comparator by comparison with a standard meter^ belong- 

 ing to Professor Rogers of Harvard Observatory, which had been 

 compared at Washington with the Coast Survey standards, as well as 

 by comparison with one of our own meter scales which had also been 

 so compared. The result was .26908 meter at 20° C. 



To this must be added the thickness of the silk tape suspending the 

 weights. This thickness was carefully determined by a micrometer 

 screw while the tape was stretched, the screw having a flat end. The 

 result was .0003 1"\ 



So that, finally, D = .26939 meter at 20° C. Separating the 

 constant from the variable parts, the formula now becomes 



J 86.324 / .0006 \ Wn 



— = — — - ( 1 + .000018 (/" — 20) + ^r — -835 a ) -^« 

 (j A -f .347 V ' v ' ~ W ' t— i 



(j = 9.8005 at Baltimore. 



It is unnecessary to have the weights exact to standard, provided 

 they are relatively correct, or to make double weighings, provided the 

 same scale of the balance is always used. For both numerator and 

 denominator of the fraction contain a weight. 



2. Experimental Data and Tables of Results. 



In exhibiting the results of the experiments, it is much more sat- 

 isfactory to compute at once from the observations the work neces- 

 sary to raise l klL of the water from the first temperature observed to 

 each succeeding temperature. By interpolation in such a table we 

 can then reduce to even degrees. To compare the different results I 

 have then added to each table such a quantity as to bring the result 

 at 20° about equal to 10,000 kilogramme meters. 



The process for each experiment may be described as follows. 

 The calorimeter was first filled with distilled water a little cooler 

 than the atmosphere, but not so cool as to cause a deposit of dew. 

 It was then placed in the machine and adjusted to its position, 

 though the outer half of the jacket was left off for some time, so that 

 the calorimeter should become perfectly dry ; to aid which the calo- 

 rimeter was polished with a cloth. The thermometer and safety- 

 tube were also inserted at this time. 



Affer half an hour or so, the chronograph was adjusted, the outer 

 half of the jacket put in place, the wooden screen fixed in position, 

 and all was ready to start. The engine, which had been running 



