238 



NATURE 



[July 22, 1875 



both voltameters is ascertained by inserting equal known resist- 

 ances in both branch circuits, when 



should be the 'result. Facing this, the bilance is generally 

 re-established by reversing the poles of the battery, the reason 

 being that hydrogen electrodes are liable to accumulate metallic 

 or other deposit upon their surfaces, which is effectually removed 

 by oxygen. 



When the instrument is to be worked between wide ranges of 

 temperature, it is requisite that C should be variable, and nearly 

 equal to X, and that 7 should be very small compared with X. 



By equating the values of the equations 



X= -(C+7)-7 = r = -0393^9^1 +-00216407/— -24127, 

 1/ 

 C and 7 in the instruments constructed being equal to 17 and 

 2 units, we arrive at 



/°Cent.r= j {877-975 X 5 + ioi-8o877)J - 9-0960553 j ' - 274', 



from which a table has been prepared to be used with the pyro- 

 meter. 



The precautions which have to be taken to insure reliable 

 results in using the Differential Voltameter are : — 



1st. The dilute acid employed in both tubes should be of 

 equal strength. * 



2nd. After disuse, the equality of the resistances of the vol- 

 tametres and connection should be verified by passing the 

 current through them with equal resistances in each branch. 



3rd. The bittery power should be proportional to the resist- 

 nnces to be measured, whilst owing to the voltameter exer- 

 cising an opposing electro-motive lorce by polarisation, less 

 than five Danieli's elements should not be employed. 



4th. The india-rubber pads should be smeared from time to 

 time with a waxy substance such as resin ceratel 



"With these precautions the measurements of the instrument 

 have been compared with a very perfect Wheatstone bridge 

 arrangement, and tables of results are given showing that it can 

 be relied upon to within one-half per cent, of error ot obser- 

 vation. Its principal advantages are stated to be : that the 

 resistance is measured in work done, and does not therefore 

 depend upon a momentary observation, that it is not influenced 

 by motion on board ship or by magnetic disturbances, and that 

 its construction is so simple that each part can be easily exa- 

 mined and verified. 



It is regarded, however, only as a useful adjunct to the more 

 important subject of thermometry, wliich forms the principal 

 object of this paper. 



THE GIGANTIC LAND TORTOISES OF THE 

 MASCARENE AND GALAPAGOS ISLANDS* 



EVER since the foundation of Natural History Col- 

 lections in Europe, naturalists had their curiosity 

 excited by shells of Tortoises of enormous size that were 

 brought home in vessels coming from India. From the 

 accounts of travellers as well as from the great convexity 

 of their shell, these tortoises were known to be terrestrial 

 in their life, and totally distinct from the other giants of 

 the Chelonian order, the marine Turtles. Various loca- 

 lities having been given as their habitat, such as the Cape 

 of Good Hope, the Coast of Coromandel, Malacca, 

 China, &c., the impression prevailed that they were 

 found in many parts of India, and consequently nothing 

 could have been more appropriate than the name given 

 to them, Testudo indica. 



It is not the object of the present article to treat in 

 detail of the divergent views held subsequently by zoolo- 

 gists, some distinguishing several species from the differ- 

 ence of the form of the shell alone, others maintaining 

 that there was one very variable species only which had 

 been carried by ships from its native place into various 

 parts of the globe where it became acclimatised, until 



* The substance of this article is contained in a paper read by the 

 author before the Royal Society in June, 1874, which will appear in the 

 ■forthcoming volume ot the " Philosophical Transactions," and to which I 

 must refer for the scientific portion and other details. Some facts which 

 have come to my knowledge subsequently to the reading of this paper, 

 Ore added. 



E)r. Gray, the principal advocate of the latter opinion, 

 himself was compelled to admit that there must be at 

 least two kinds, one with a convex and the other with a 

 ilat skull. The scientific study of these tortoises may b2 

 said to have commenced with this distinction, but it com- 

 menced at a time when the work of disturbance and 

 extermination by man had already reduced the amount of 

 evidence so far as to well nigh bring the subject into the 

 domain of pateontological research. 



From the accounts of voyagers of the sixteenth and 

 seventeenth centuries we learn that these tortoises were 

 found at two most distant stations, one being the Gala- 

 pagos group in the Pacific, the other comprising some of 

 the islands of the Indian Ocean ; Mauritius, Rodriguez, 

 Aldabra, and probably Rdunion. Widely different as 

 these stations are in their physical characteristics, they 

 had that in common, that they were, at the time of their 

 discovery, uninhabited by man or even by any large 

 terrestrial mammal. There is not the slightest trace of 

 evidence that any of the intervening lands or islands have 

 ever been inhabited by them. 



At first the Tortoises were found in immense numbers 

 and of extraordinary size. Leguat (1691) says that in 

 Rodriguez " you see two or three thousand of them in a 

 flock, so that you may go above a hundred paces on their 

 backs ;" and indeed, when we consider that these helpless 

 creatures lived for ages in perfect security from all 

 enemies, and that nature has endowed them with a most 

 extraordinary degree of longevity, so that the individuals 

 of many generations lived simultaneously in their island 

 home, we can well account for the multitudes found by 

 the first visitors to those islands. For a period of more 

 than a century they proved to be a source of great 

 benefit to the crews and passengers of ships, on account 

 of their excellent and wholesome meat. In times when a 

 voyage, now performed in a few weeks, took as many 

 months, when every vessel, for defence's sake and from 

 other causes, carried as many people as it was possible 

 to pack into her,- when provisions were rudely cured and 

 but few in kind, these tortoises which could be captured 

 in any number with the greatest ease in a few days, were 

 of the greatest importance to the famished and scorbutic 

 ship's company. The animals could be carried in the 

 hold of the ship for many months without food, and were 

 slaughtered as occasion required, each tortoise yielding 

 from 80 to 300 pounds of fresh wholesome food ; and we 

 read that ships leaving the Mauritius or the Galapagos 

 used to take upwards of 400 of these animals on board. 



Although no account of the first discovery of the 

 Galapagos Islands appears to have been published, so 

 much is certain that it is due to the Spaniards, who 

 applied the Spanish word for tortoise to this group of 

 islands. It became the regular place of meeting and re- 

 fitting to the buccaneers and whalers, who provisioned 

 themselves chiefly with tortoises and turtles. But nume- 

 rous and constant as these visits were, the reckless de- 

 struction of animal life was limited chiefly to the coast- 

 belt, and numbers of the animals inhabiting the interior 

 escaped ; no regular or extensive settlement being 

 attempted, . the condition of the islands and of the 

 animals inhabiting them remained in the main un- 

 altered until the earlier portion of the present century. 

 From the accounts of that period I select that given 

 by Porter, a Captain in the United States Navy, as the 

 one which contains by far the most interesting ob- 

 servations (Journal of a cruise made to the Pacific 

 Ocean, New York, 1822, 8°). He found, in the year 

 1 8 13, the tortoises in greater or less abundance in all 

 the larger islands of the group which he visited, 

 viz., Hood's, Narborough, James, Charles, and Porter's 

 Islands. On Chatham Island he found only a few of their 

 shells and bones, which appear to have been lying there for 

 a long time, and possibly may have belonged to indi- 

 viduals transported from some other island. On Albe- 



