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Part TIL — Eighteenth An aval Report 



Measuring Board. 



In order to readily obtain the points at which the perpendiculars 

 from the different points cut the lateral axis, a board (vide figs. 1 and 2), 

 which served this purpose with a considerable amount of accuracy, 

 was constructed. The apparatus was furnished with a movable frame, 

 carrying a thread which cut the lateral axis at right angles. This 

 frame could be moved from one end of the board to the other, and could 

 thus be brought exactly over each selected point in succession. The two 

 supports which received the ends of the thread were connected together 

 by a wooden cross-piece that held them firmly united. The near support 

 was shaped as the base of a T square, so that when it was pushed up 

 against the edge of the board the thread crossed the same at right angles. 

 The edge of this support moved over a millimetre scale. A needle point 

 was inserted in this edge, immediately below the thread. The frame was 

 moved along until the pin that marked the point on the fish, the thread, 

 and the needle point were in the same plane. The distance was, with the 

 aid of the needle point, then read off on the scale. The thread could be 

 raised or lowered to suit the varying thickness of the fish. At either end 

 of the board there was a support which received one end of a thread 

 which traversed the board longitudinally. This represented the lateral 

 axis of the fish, and was drawn parallel to the edge of tlfe board. The 

 lateral axis was also represented by a groove cut in the board itself. The 

 fish, when ready for measurement, was placed on the board with the 

 anterior tip of the mandible and the middle of the fork of the tail exactly 

 on the lateral axis, so far as could be judged by the eye. The tip of the 

 mandible was in contact with the upright at the zero point of the scale. 

 The distances were accurately measured to half a millimetre. The 

 measurements of the skull, mandible, and dentary were made by means 

 of callipers, the fish having been first boiled. Other dimensions — e.g., 

 length of tail, etc. — were got by the aid of dividers. It has been necessary 

 to go into a minute description of the modes of measurement, for it is 

 very evident that unless there is uniformity in taking the dimensions no 

 comparison of any value can be made between their Means. 



The mackerel were sent from the West Coast to Aberdeen in ice, and, 

 with the exception of a few which were examined immediately, were put 

 into the cold-storage room of a local ice-factory. There they were kept 

 until they were required. The Aberdeen fish were similarly stored. 



Shortcomings in respect to the Measurements. 



It was not possible to measure every one of the characters which appear 

 in the list given above on each fish. Some of the fishes, through 

 damage or contortion, were unsuitable for certain measurements, and, as 

 will be seen from the Tables appended, the number of fishes in which each 

 character was noted varies considerably. In the case where the fish is 

 damaged, or is much contorted, the measurements which would be affected 

 thereby can be neglected, but where the fish does not show any sign of 

 damage there may, however, be a certain amount of contortion, which will 

 affect the accuracy of the measurements. This slight contortion, which is 

 not apparent, and which usually occurs at the neck or in the anterior 

 part of the trunk, may have a considerable effect on the disposition of the 

 soft parts, and consequently on such characters as the distance of the fins 

 from the tip of the snout. The contortion has the effect of shortening 

 these distances. The true distance of any of these characters is to be got 

 only when the dorsal fins, tail fin, and centre of the tip of the snout are 

 in the same plane, which, when the fish is swimming, is vertical. Any 



