116 THE ROYAL SOCIETY OF CANADA 
15 in all, being given in the tables of displacements following. From 
the 4th to the 13th inclusive, are the lines selected for measurement 
by the Rotation Committee; the first three and the last two are lines 
used by Adams, which are of interest on account of giving a different 
velocity value from the general reversing layer. The measurements 
were all made on a special Toepfer measuring engine having a micro- 
meter screw of 0.5 mm. pitch, and 300 mm. long. The head reads 
directly to thousandths and by estimation to ten thousandths of a milli- 
metre. The plates were measured in the usual way by making 4 settings 
on the central strip, and 2 each on top and bottom strips. The difference 
of the means is the displacement. When the measure is completed 
the plate is reversed on the carriage and remeasured as before. Thus 
each displacement is the mean of 8 settings. There is some indication 
of a slight systematic difference in the displacements obtained in the 
two positions of the plate, sufficient to indicate the desirability of measur- 
ing both ways. In any case the lines frequently have a different 
appearance on reversal and no doubt a much more accurate 
value of the displacement is obtained. The displacements and probable 
errors in the following tables are given in ten thousandths of a milli- 
metre. 
DISPLACEMENTS. 
(Measured by Plaskett.) 
| Plate No. 699. Plate No. 701. 
Line a b e d e f a b cd e f 
4196-699...... | 683 | 692| 710| 704| 709| 706| 693) 717| 683} 693] 711] 704 
4197-257...... 706 | 716) 714| 705} 719} 700| 717 | 702) 715} 696 | 681) 699 
4216-1862 688 | 710 | 689 | 696 | 706 | 702| 704] 709 | 699| 689 | 703 | 705 
4220-509...... 686 | 693] 705| 688 | 695] 704} 710| 694] 692} 696 | 706| 709 
4225-619...... 703 | 688| 700! 686 | 705| 704| 696 | 708 | 710 | 696 | 683 | 699 
41222-88704 | 689 | 677 | 695| 704| 684] 696| 715| 707 | 703 | 689 | 705| 695 
4241-2850... | 692| 708 | 704 | 696 | 691 | 710| 710 | 711 | 707 | 702 | 689 | 716 
4246-996...... | 689| 675| 693 | 682 | 695| 700| 707 | 699 | 701 | 685 | 687 | 689 
4257 -815:.:... 702 | 675] 693 | 675| 717 | 708| 713 | 711| 713 | 700 | 698 | 698 
4258" 271.1. 691 | 704| 697 | 691 | 691 | 703| 715| 705| 716 | 689 | 685| 701 
4266-081...... 679 | 706| 721 | 682| 707 | 712| 698 | 706 | 721 | 700 | 687 | 689 
4268-915...... 681 | 704| 698 | 682 | 717 | 707| 712| 711 | 708 | 696 | 696 | 710 
4276-836...... 691 | 691] 708 | 684| 700 | 686| 711] 705| 698 | 702| 712| 709 
4290-377...... 699 | 686| 705| 699 | 706 | 696| 703| 695| 713| 693 | 709| 704 
4291-630...... | 700 | 681] 695 | 686 | 715 | 710! 713] 709 | 711] 684] 706| 700 
Mean Disp..... 692 693 702 691 704 703 708 706 706 694 697 702 
P.E.Single Line 9-1 6-0 6-4 7-2 4-6 5:0 4-2 6-8 3-9 7-4 5-0 5:6 
Average probable error single line............ + -00059 mm. 
» * AR Se ON EP aoe ach + «00016 
: 't MÉBDISLE RER Re re vide goes oe «0700  “ 
Probable error displacement, single plate....... == -00041 “ 
