124 PROCEEDINGS OF THE AMERICAN ACADEMY. 



the method of mounting and using the disk now under consideration. 

 Certain small changes, however, must be imagined in Figure 2, in order 

 to make it accord perfectly with the latest developments of the apparatus. 

 Thus, the water stream entering beneath the disk is no longer allowed to 

 flow without restriction straight against the centre of the lower face. A 

 little affair shaped somewhat like a three-legged stool is placed at the top 

 of the admission tube, just beneath the point marked by the letter C, 

 and most of the water escapes laterally between the legs of the stool, 

 although a small part of it runs through a hole in the top straight against 

 the disk. This device was adopted in the hope that it would make the 

 temperature more uniform over the lower face of the disk. With a 

 similar purpose regarding the upper face of the disk, the hard rubber 

 block HH has been replaced by one having a somewhat more gradual 

 curvature, so as to make a thinner and more rapid stream over the 

 central parts of the disk. It is doubtful whether these changes have 

 done much good, on the whole, although they appear to have made the 

 difference of temperature between top and bottom of disk at the centre 

 very nearly equal to the mean difference at other parts, as numbers 

 presently to be given will show. Another attempt to improve the distri- 

 bution of temperature, by making the flow of water more nearly equal 

 along different radii of the upper face of the disk, affected the manner of 

 admission of the water to the funnel FF ; but, as it was of doubtful 

 utility, it need not be described. The air-vent leading up from the 

 funnel FF by the tube w has been considerably enlarged ; but the small 

 escape of water which had previously been maintained at this vent is no 

 longer permitted. 



The small copper wires, extending from the copper coatings of the 

 disk, are now protected fi'om actual contact with the hard rubber plugs 

 Ki, K2, etc., and with the soft rubber packing surrounding these plugs, 

 by a wrapping of oiled silk, with the purpose of preserving tlie wires from 

 the destructive action of the sulphur emitted from the rubber ; but, after 

 all, the wires cannot be depended upon for more than a few months. 



Stops about 0.16 cm. thick, placed at the edges of the faces of the 

 disk, prevent the blocks HH and H'H' from approaching these faces so 

 near as to endanger the safety of the small wires or cut off the flow of 

 water. 



The parts marked Jj and Jo in Figure 2 of the former paper now 

 contain spirals of platinum instead of thermo-electric junctions of copper 

 and German silver, change of resistance of platinum having been substi- 

 tuted for change of thermo-electromotive force as a means of measuring 



