AT KEW OBSERVATORY AND THEIR DISCUSSION. 443 



soon becomes apparent. Again, on allowing the pressure to rise to its original value, 

 one finds the aneroid read lower than at the start, this depression gradually 

 disappearing. The instrument, in fact, behaves as if an imperfectly elastic body. 

 The strain, under a uniform stress, tends to increase, and as a natural concomitant, 

 there is elastic after-effect (elastische Nachwirkung). That the whole of the difference 

 between readings with pressure descending and ascending (or, as we shall call them 

 for brevity, descending and ascending readings) represents, in all cases, a true after- 

 effect, I am not prepared to say. If we regard a rise of temperature as analogous to 

 a fall of pressure, a thermometer presents a somewhat parallel case ; and we know 

 that there the difference between ascending and descending readings is due to at 

 least two causes, viz., change in the mercury meniscus, with consequent alteration of 

 internal pressure, and temporary change of zero following exposure to the higher 

 temperature. Of these two causes, the latter only is a true after-effect ; but the 

 former may also conceivably have its counterpart in the mechanism of the aneroid. 



An aneroid showing large after-effect is not a suitable instrument for travellers. 

 Readings taken with it, for instance, during a mountain descent, are largely influenced 

 by the elevation of the summit and the time spent there. Thus, from the outset. I 

 have regarded the after-effect phenomena as specially requiring investigation. 



3. As a first step, I examined the records of about 300 aneroids, tested over the 

 ranges 30-15, 30-18, 30-21, 30-23, 30-24, and 30-2G inches. The differences 

 between the descending and ascending readings were noted for the several ranges, 

 and the general character of the phenomena ascertained. There existed, however, no 

 data for connecting the results from the several ranges, it being customary to test 

 each aneroid over one range only. Special experiments were thus requisite, in which 

 the same aneroids should be taken over a variety of ranges. For such a purpose it 

 was impossible to employ the ordinary working apparatus, with a due regard to the 

 regular work of the Observatory. Application was accordingly made by the Kew 

 Observatory Committee to the Government Grant Fund, and 30 was obtained for a 

 new air-pump and receiver, and for a set of aneroids to be experimented on. 



The only special feature of the apparatus* is a second or auxiliary receiver, between 

 the pump and the main receiver which contains the aneroids. On the tube connecting 

 the two receivers are two stop-cocks, a third cock being placed between the auxiliary 

 receiver and the pump. One can thus exhaust the auxiliary receiver separately, and 

 then by manipulating the cocks lower the pressure in the main receiver at any 

 desired rate. This arrangement avoids the sudden changes of pressure to which 

 aneroids are exposed when in a receiver connected directly to the pump. The pump, 

 the receivers, and the mercury gauge which shared the pressure of the main 

 receiver were rigidly attached to a stout board which rested on a table. The 



* The apparatus was obtained from Mr. J. J. HICKS, 8, Hatton Garden, London. It was solidly 

 constructed and proved very satisfactory. The experimental aneroids were likewise purchased from 

 Mr. HICKS. 



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