A DESCRIPTION OF THE DIVIDING ENGINES DESIGNED 

 BY PROFESSOR ROWLAND. 1 



Three dividing engines were made under Professor Rowland's direc- 

 tion, all embodying the same general principles as given in his article 

 on the " Screw " in the Encyclopedia Britannica (this volume p. 506). 

 The screws of all three have approximately twenty threads to the inch; 

 and the number of teeth in the ratchet wheels of the first, second and 

 third machines is such that they rule 14,438, 15,020 and 20,000 lines in 

 an inch. The three machines are kept in the sub-basement of the 

 Physical Laboratory of the Johns Hopkins University under such con- 

 ditions as will secure a practically constant temperature for long 

 intervals of time. Each machine is driven by a separate water-motor 

 whose speed can be regulated at will. 



The machines have been used almost exclusively for the ruling of 

 Diffraction gratings, although a few centimetre scales have been made. 

 The gratings have been, with only four or five exceptions, made of 

 " speculum metal/' having the composition, copper 126 Ibs. 4 oz., tin 

 58 Ibs. 9 oz., and as homogeneous as possible. The rough metal plates 

 were cast under Professor Rowland's direction, and were then figured 

 and polished. After the ruling was completed, the gratings were care- 

 fully tested in order to see if they were free from "ghosts," diffused 

 light and defective definition. 



To test the screw, ratchet-head and thrust screw for periodic errors, 

 P-ofessor Rowland used the following method: he ruled a space of 

 about one centimetre on a polished surface, then pushed the carriage 

 ba-'k this distance, turned the grating-holder through a minute angle 

 ai again ruled a surface of about the same width as before. There is 

 th - produced a cross-ruling, the lines being slightly inclined to each 

 ot;'?r; and when examined by reflected light, a series of undulations is 



1 Unfortunately Professor Rowland never published a description of these machines ; 

 &nC the Committee has failed to find any information concerning the inception of 

 the idea or the history of the construction of the first machine. It has been 

 thought best, therefore, to give, first, a general description of the design of the 

 engines with various necessary details of some of the working parts and, second, 

 drawings made to scale, showing all the connections of the intricate mechanism ; 

 both of these have been prepared under the direction of J. 8. Ames, Secretary of 

 the Committee, and have been approved by the Committee. 



