552 REPORTS ON THE STATE OF SCIENCE. — 1910. 



3. The Aneroid Barometer. 



This instrument was constructed by the Cambridge Scientific Instru- 

 naent Company at very short notice. It was then kindly placed at the 

 disposal of the writer by Mr. Horace Darwin. Fig. 1 represents its main 

 features. 



The series of boxes B mounted on the horizontal axis A is suspended 

 by the thin steel springs CC, 0-45 mm. thick, from a pair of square bars 

 shown in section, which are supported by pairs of pillars DD mounted 

 on a solid metal base BE. The axis is under no other constraint, but a 

 loose link, pointed at each end, and fitting into cones on either side, makes 

 a connexion between it and the screw which passes through one end of the 

 base. The extension of the boxes, which is a measure of the atmospheric 

 jn-essure, is measured by finding the amount through which the divided 

 head H must be turned in order to press the other end of the axis against 

 the stop G. Contact is determined by flicking a spring along one side of 

 which, at a distance of 7-5 cms. froni the support, the contact piece G is 

 fixed, and as the screw head is slowly turned, noting the instant at which a 

 tinkle indicates that the axis is in contact with the vibrating spring. If 

 the boxes pump there is a tendency for the contact to be registered too 

 early, which gives too low a value for the pressure. 



To obviate the effects of the ship's motion, Mr. Horace Darwin devised 

 the suspension also shown in fig. 1 . 



The box K, containing the instrument, was susjiended by springs from 

 three arms at 120° with one another, which were united over a central 

 jnyot sujjported upon a frame fixed to a firm base. A dash-pot L con- 

 taining oil and a plunger to damp the vertical vibrations was fixed to the 

 top of the instrument case. 



During the voyage a level and sliding weights were added and a 

 thermometer arranged to record the temperature within the case. The 

 instrument gave very steady readings (consistent to 0025 millibars at 

 land stations), but at sea the pumping was unfortunately wqyj apparent. 

 The calibration of the instrument in a closed chamiae/ is difficult, as 

 it requires two hands for every manipulation, one for turning the milled 

 head and the other for flicking the spring by a device which extends 

 through the wooden side of the box. 



Though, as will eventually appear, the instrument has certain failings, 

 its investigation at sea has brought to light certain ))oints which will 

 prove of value to the future designing of an instrument of sufficient delicacy 

 for the object of this research. 



4. The Observations. 



As explained in the Interim Repoi-t (1915), the apparatus was carried 

 in s?. Ascaiuus on the outward and in R.M.S. Morea on the homeward 

 voyage. Observations were made three times a day as a rule during both 

 voyages. Each set of observations consisted of the following : — 



(1) Reading of temperature of special chamber from outside by thermo- 

 meter projecting through wall. 



(2) Entering chamber and closing door quickly. 



(3) Temperature of aneroid and five observations of the pressure 

 recorded. 



