302 



NATURE 



{Feb. 17, 1876 



earth's magnetism is a maximum near the solstices, and 

 a minimum near the equinoxes. 



The diurnal variations of horizontal force, deduced 

 from the photographic registration of the bifilar magneto- 

 meter, follow laws similar to those at Munich, and not 

 differing greatly from those at Makerstoun, 17° further 

 north ; the minimum, however, shifting from near 9 A.M. 

 in summer to about 2 P.M. in winter. 



The balance magnetometer seems to be the least certain 

 of the variation instruments at Lisbon. The temperature 

 coefficient (obtained by heating the air with gas jets) has 

 been found with an opposite sign to that due to varia- 

 tions of the needle's magnetism, a result which is always 

 unsatisfactory even when the variations of temperature 

 are small as they are at Lisbon. The diurnal variation 

 of the vertical magnetic force differs considerably from 

 that obtained at more northern observatories, the mini- 

 mum occurring in each month of the year near noon, and 

 the maximum near 5 P.M. 



Mr. Capello has evidently bestowed much pains on the 

 determination of his instrumental constants, and this pub- 

 lication of results contains a valuable contribution to our 

 knowledge of the magnetic laws for an important station, 

 near the most southerly and westerly point in Europe. 

 Lisbon, like nearly every other magnetic observatory, has 

 been obliged to be satisfied with single instruments of 

 each kind. When so many observatories were founded 

 between thirty and forty years ago, there was perhaps an 

 over confidence in the excellence of the instruments em- 

 ployed, and in the certitude of being able to correct 

 the observations to be obtained from them for every 

 possible error. There was also the economical considera- 

 tion connected with the expense of a double series of 

 instruments, as well as the additional labour incurred in 

 observing two instruments for the same purpose. The 

 consequence has been, to take a single illustration, that 

 no two observatories have given exactly the same law for 

 the annual variation of the mean position of the magnetic 

 needle. One observatory has contradicted another, the 

 results from a good instrument have been balanced by 

 those from a bad one, and in other cases it has not been 

 possible to determine whether the differences found at two 

 stations were really due to difference of locality only, or to 

 instrumental causes. 



When we remember the vast labour (to omit every 

 other consideration) expended in obtaining the laws of 

 magnetic variations, it cannot be too much regretted that 

 every observatory was not furnished with a double series 

 of instruments, which would have shown by their agree- 

 ment or disagreement the accuracy or error of the results 

 obtained from them. In the case of disagreement the 

 director of the observatory would have been warned that 

 some error existed whose cause should be sought out. 

 No preliminary trials can ensure that an instrument will 

 remain with exactly the same errors. If we could sup- 

 pose that the captain of a ship would set sail on a lengthy 

 and costly voyage with a single chronometer, without any 

 means of verifying the accuracy of its going except the 

 meeting with another ship in a like predicament, and 

 should then find that, according to their chronometers, 

 they were on opposite sides of the globe, we should have 

 a parallel to a not uncommon case in the work of many 

 magnetic observatories. 



It is to be hoped for the future that such differences 

 will not be allowed to exist, that each observatory will 

 have the means of proving that, for its locality at least, 

 the laws obtained are true, and that in publishing the 

 observations, the differences of the indications of two 

 instruments of each kind will be given with the most com- 

 plete exposure of their errors and corrections. 



John Allan Broun 



MARSDEN'S "NUMISMATA ORIENTALIA" 

 M arsden^ 5 International NtimismataOrientalia. Part II. 

 Coins of the Ursuki Turkomans. By Stanley Lane 

 Poole, Corpus Christi College, Oxford. (London : 

 Triibner and Co., 1876.) 



THIS is the second part of the series of separate 

 publications on the Early Coins of the East, of 

 which the first part on Ancient Indian Weights was 

 reviewed in Nature, vol. xii. p. 24. The whole work is 

 intended to be a new edition of Marsden's " Numismata 

 Orientalia," but in consequence of the new form of the 

 work and its enlarged character, the editor has changed 

 its title into that of the " International Numismata 

 Orientalia." 



Part II. has been undertaken by Mr. Stanley Lane 

 Poole, and treats of the Coins of the Ursuki dynasty. 

 Ursuk was one of the petty chiefs of Syria during the 

 wars of the Crusades, in which he distinguished himself, 

 and was made Governor of Jerusalem, A.D. 1086. His 

 descendants, the Ursuki princes, were amongst the most 

 powerful chiefs in Syria and Mesopotamia, until the 

 dynasty was brought to a close by the Tartar invaders, 

 A.D. 1242. An historical sketch of the Ursuki family is 

 given by Mr. Poole as an introduction to the account <rf 

 their coins. 



The series of Ursuki coins described in the work, the 

 greater part of which are nowj in the British Museum, 

 are mostly copper coins, a few only being of silver. 

 Several plates with clear lithographic and photographic 

 representations of the coins form part of the work. The 

 coins all bear Arabic inscriptions, some of considerable 

 length, and they appear to be of much historical value. 

 In the description of the several coins in the text of the 

 work the old Arabic inscriptions on each coin are given 

 in the more modern Arabic character, according to the; 

 system of transliteration adopted in the book. These; 

 inscriptions are, however, intelligible only to Arabic 

 scholars, as no English translation is given, which would; 

 have added considerably to the interest of the work for 

 general readers. 



At the commencement of the Ursuki dynasty the Ma- 

 hometan moneys were of three classes— gold, silver, and 

 copper, the respective units being the dinar, dirlwi: 

 and fels. But the Ursuki coins, both copper and silver 

 appear to be dirhems, this word appearing in the inscrip 

 tionof many of the coins, and showing that they wert 

 intended to pass as dirhems. Some of the copper coin; 

 have a thin coating of silver, and one has been gilded. 



The Ursuki princes were amongst the few Mahometar 

 dynasties that introduced images on their coins. Bu 

 they rarely, if ever, engraved their own heads or those 

 their suzerains on their coins, choosing instead the type: 

 of the gold coins either of the Byzantine Emperors or 

 the Greek Kings of Syria. 



I 



