Jan. II, 1877] 



NATURE 



^2^ 



I 



pallet, and the nut-collar screws down as much as it 

 rose during the preceding interval of freedom when 

 the action is regular ; and the central or main escape- 

 ment-shaft turns in the same period as the tooth, being 

 the period of the pendulum. If through increase or 

 diminution of the driving-power, or diminution or in- 

 crease of the coefficient of friction between the governing 

 masses and the ring on which they press, the shaft tends 

 to turn faster or slower, the nut collar works its way down 

 or up the screw, until the governor is again regulated, 

 and gives the same speed in the altered circumstances. 

 It is easy to arrange that a large amount of regulating 

 power shall be implied in a single turn of the nut collar 

 relatively to the central shaft, and yet that the periodic 

 application and removal of about :^^^ of this amount in the 

 half period of the pendulum shall cause but a ve/y small 

 periodic variation in the speed. The latter important 

 condition is secured by the great moment of inertia of the 

 governing masses themselves round the main shaft. My 

 communication to the Royal Society ended as follows : — 



" I hope after a few months' trial, to be able to present 

 a satisfactory report of the performance of the clock now 

 completed according to the principles explained above. 

 As many of the details of execution may become modified 

 after practical trial, it is unnecessary that I should de- 

 scribe them minutely at present. Its general appearance, 

 and the arrangement of its characteristic parts, may be 

 understood from the photograph now laid before the 

 Society." 



I am sorry to say that the hope here expressed has not 

 hitherto been realised. Year after year passed producing 

 only more or less of radical reform in various mechanical 

 details of the governor and of the fine movement, until 

 about six months ago, when, for the first time, I had all 

 except the pendulums in approximately satisfactory con- 

 dition. By that time I had discovered that my choice of 

 zinc and platinum for the temperature compensation, and 

 lead for the weight of the pendulums, was a mistake. I 

 had fallen into it about ten years ago through being in- 

 formed that in Russia the gridiron pendulum had been 

 reverted to because of the difficulty of getting equality of 

 temperature throughout the length of the pendulum ; and 

 without stopping to perceive that the right way to deal 

 with this difficulty was to face it and take means of secur- 

 ing practical equality of temperature throughout the length 

 of the pendulum (which it is obvious may be done by 

 simple enough appliances), I devised a pendulum in which 

 the compensation is produced by a stiff tube of zinc and a 

 platinum wire placed nearly parallel each to the other 

 throughout the length of the pendulum. The two pen- 

 dulums of the clock shown to the British Association were 

 constructed on this plan. Now it is clear that the mate- 

 rials chosen for compensation should, of all those not 

 otherwise objectionable, be those of greatest and of least 

 expansibility. Therefore, certainly, glass or platinum 

 ought to be one of the materials, and the steel of the ordi- 

 nary astronomical mercury pendulum is-a mistake. Mer- 

 cury ought to be the other (its cubic expansion being six 

 times the linear expansion of zinc) unless the capillary un- 

 certainty of the mercury surface lead to irregular changes 

 in the rate of the pendulum. The weight of the pendulum 

 ought to be of material of the greatest specific gravity 

 attainable ; at all events unless the whole is to. be mounted 

 in an air-tight case ; because one of the chief errors of the 

 best existing pendulums is that depending on the varia- 

 tions of barometric pressure. The expense of platinum 

 puts it out of the question for the weight of the pendulum, 

 even although the use of mercury for the temperature 

 compensation did not also give mercury for the weight. 

 Thus even though as good compensation could be got by 

 zinc and platinum as by any other means, mercury ought 

 on account of its superior specific gravity (nearly three 

 times that of lead) to be preferred to lead for the weight 

 of the pendulum. 



I have accordingly now made several pendulums (for 

 tide-gauges) with no other material in the moving part 

 than glass and mercury, and with rounded knife edges of 

 agate for the fixed support ; and I am on the point of 

 making four more for two new clocks which I am having 

 made on the plan which forms the subject of this com- 

 munication. I have had no opportunity hitherto of test- 

 ing the performance of any of these pendulum^, but their 

 action seems very promising of good results, and the 

 only untoward circumstance which has hitherto appeared 

 in connection with them has been breakages of the glass in 

 two attempts to have one carried safely to Genoa for a tide- 

 gauge made by Mr. White, to an order for the Italian 

 Government. 



As to the accuracy of my new clock, it is enough to 

 look at the pendulum vibrating with perfect steadiness, 

 from month to month, through a range of half a centimetre 

 on each side of its middle position, with its pallets only 

 touched during ^J^ of the time by the escapement-tooth, to 

 feel certain that, if the best ordinary astronomical clock 

 owes any of its irregularities to variations of range of its 

 pendulum or to impulses and friction of its escapement- 

 wheel, the new clock must, when tried with an equally 

 good pendulum, prove more regular. I hope soon to 

 have it tried with a better pendulum than that of any 

 astronomical clock hitherto made, and if it then shows 

 irregularities amounting to yg of those of the best astro- 

 nomical clocks, the next step must be to inclose it in aa 

 air-tight case kept at constant temperature, day and night, 

 summer and winter. 



ON THE TROPICAL FORESTS OF 

 HAMPSHIRE ' 



ENGLAND at the present time has a climate far from 

 tropical, but at the time to which this lecture refers 

 the palm and spice plants flourished here ; and hence the 

 climate then may rightly be spoken of as actually tropical. 



The data on which this inference is based are the fossil 

 leaves which are found in the clays of the south of Hamp- 

 shire. Out of the many thousands of such leaves obtained 

 by me during summer holidays for many years past, some 

 selected specimens were exhibited in a cabinet in the Loan 

 Collection of Scientific Instruments. Other collections of 

 leaves from this spot and from Alum Bay have been made, 

 and may be seen in the British Museum. It is the district 

 immediately along the line east and west of Bournemouth 

 which has been specially examined, and it is in the lower 

 Bagshot beds, which are, comparatively speaking, amongst 

 the youngest of the geological scale, that the leaves re- 

 ferred to have been found. 



These Bagshot beds need not detain us ; but as I have 

 referred to them as amongst the youngest in the geological 

 scale, I may mention that above them we have the Brackle- 

 sham beds, full of marine forms ; the Barton beds, also full 

 of marine forms, but telling a tale of a different sea ; the 

 Headon, Bembridge, and Hempstead series, with many 

 repetitions of marine and fresh-water conditions, indicating 

 long lapses of time. There is, too, the whole Miocene 

 period, of which we have no trace in this district, but which 

 we believe from continental evidence was of vast duration. 

 Then, too, there followed periods of immense length, 

 during which England underwent its latest glacial epoch ; 

 after that, the time during which the gravels were formed. 

 While, therefore, we speak of these beds as almost the 

 youngest of our series, they belong to periods of an in- 

 calculably remote past. 



It is from the cliffs principally, and from the deep 

 cuttings of the recently constructed railway from Bourne- 

 mouth to Parkstone, that our knowledge is mainly de- 

 rived. There are, in addition, the diggings carried on 



I Lecture in connection with the Loan Collection of Scientific Apparatus, 

 given at the South Kensington Museum, December 3 1876, by J. Starkio 

 Gardner, F.G.S. 



