ON THE MECHANICAL EQUIVALENT OF HEAT 



361 



The Cathetometer 



The cathetometer was one made by Meyerstein, and was selected 

 because of the form of slide used. The support was round, and the 

 telescope was attached to a sleeve which exactly fitted the support. 

 The greatest error of cathetometers arises from the upright support 

 not being exactly true, so that the telescope will not remain in level 

 at all heights. It is true that the level should be constantly adjusted, 

 but it is also true that an instrument can be made where such an ad- 

 justment is not necessary. And where time is an element in the 

 accuracy, such an instrument should be used. In the present case it 

 was absolutely necessary to read as quickly as possible, so as not to 



FIG. 3. 



leave time for the column to change. In the first place the round 

 column, when made, was turned in a lathe to nearly its final dimen- 

 sions. The line joining the centres of the sections must then have 

 been. very accurately straight. In the subsequent fitting some slight 

 irregularities must have been introduced, but they could not have been 

 great with good workmanship. 5 The upright column was fixed, and 

 the telescope moved around it by a sleeve on the other sleeve. Where 

 the objects to be measured are not situated at a very wide angle from 

 each other, this is a good arrangement, and has the advantage that any 

 side of the column can be turned toward the object, and so, even if it 



4 The change of level along the portion generally used did not amount to more 

 than -1 of a division, or about -Olmm. at the mercury column, as this is about the 

 smallest quantity which could be observed on the level. 



