October 28, 1909J 



NA TURE 



517 



the corpuscular radiation is small. For a slightly more 

 penetrating primary beam a rapid increase in the intensity 

 of both the secondary Rontgen radiation and the corpuscular 

 radiation takes place. This seems to suggest that the 

 production of corpuscular radiation is in some way in- 

 timatelv associated with the emission of the Rontgen type 

 of radiation. 



(2) I had recently shown that when homogeneous radia- 

 tion falls upon a thin layer of a substance which may act 

 as a secondary radiator, a portion is transinitted unchanged, 

 and that the fraction of the remaining energy which is 

 transformed into secondary Rontgen radiation decreases as 

 the primary beam becomes more penetrating. In the 

 present experiments it is found that the corresponding frac- 

 tion of the remaining energy which is transformed into 

 corpuscular radiation iucreascs as the primary beam be- 

 comes more penetrating. 



(3) The corpuscular radiation emitted by these metals 

 when subjected to homogeneous beams is itself surprisingly 

 homogeneous, whether the exciting beams are " soft " or 

 very " hard." 



(4) The absorption coetflcients of the corpuscular radiation 

 from a given metal excited by homogeneous secondary 

 Rontgen radiation vary with the nature of the exciting 

 radiation. These absorption coefficients are a decreasing 

 linear function of the atomic weight of the secondary 

 radiator. 



I hope to publish further details of these experiments 

 shortly. Ciurles A. S.udler. 



George Holt Physics Laboratory, 

 Liverpool University. 



lished in 1571, with a second edition in 1591, wherein the 

 " composition of the instrument called Theodelitus " is 

 represented as a " circle diuided in 360 grades or degrees, 

 o.' a semi-circle parted in 180 portions " ; or again, " the 

 composition also of the Square and Planisphere or Circle 

 named Theodelitus for measuring lengthes, breadths, and 

 distances." It had a " double scale," an " index," " the 

 sightes," and the circle was 2 feet in diameter, and 

 " fastened in the top of some staffe." He does not state 

 how the name was derived, and spells it " theode- 

 litus " and " theodolitus " alternately. William Bourne 

 (" Treasure for Travailers," 1578) named the same instru- 

 ment " horizontall or flatte sphere," and not theodelitus; 

 but when he speaUs of the alidade he calls it only once 

 alidi'day, but otherwise always athelida. After this 

 de Morgan, who first discussed the derivation in the 

 Philosophical Magazine, concluded that the " theodelited 

 i circle " of Digges, who, however, does not use that 

 I adjective, and " athelidated circle " of Bourne, were 

 various corruptions of the Arabic word al-idh&da (a sort 

 of rule), from which the word alidade, which carries the 

 sights or telescope of a theodolite, is derived. 



It has been suggested by various writers that theodolite 

 is derived from the Creek roots Bid. (.sight), b^6s (the way), 

 and KlBos (a stone), for the latter root Aircis (smooth) being 



Drought in South-west Ireland. 



The deficiency of rainfall in the south of Ireland, to 

 which Mr. Armstrong refers in N.^TURE of October 21 

 (p. 487), has been apparent in the annual total rainfall for 

 the last three years, the deficiency also affecting the south- 

 west of England. At the same time, there has been a 

 marked excess of rainfall in the north of Ireland, deficiency 

 and excess being taken as synonymous with quantities 

 below and above the average of many years. It is fre- 

 quently found that parts of the country, often quite narrow 

 strips, show a marked deficiency of rainfall for several 

 successive years, and afterwards revert to an average con- 

 dition or show an excess. The most probable explanation 

 seems to me to be a change, perhaps a slight one, in the 

 prevailing tracks of the centres of barometric minima, but 

 I have not found data in a form suitable for testing the 

 truth of the suggestion. 



The extreme dryness of August was experienced over a 

 large area of the south of Ireland, less than half an inch 

 of rain having fallen over about 2800 square miles. In 

 September less than half an inch fell over not more than 

 500 square miles. 



I mav perhaps be excused for pointing out that while 

 Mr. Armstrong uses " absolute drought " to describe a 

 period of twenty-four hours without rain, it has been usual 

 for many years to reserve the words " absolute drought 

 for a period of more than fourteen consecutive days without 

 recorded rainfall. Hrcii Robert Mill. 



62 Camden Square, London, N.W., October 25. 



Derivation of the Word " Theodolite." 



Although the etymology of the word theodolite 



has 



been discussed from time to time,' no satisfactory solution 

 has hitherto been established. It was first used in 

 England, and the earliest reference to it is contained in a 

 book by Leonard Digges (completed and published by his 

 son, Thomas) called " Geometrical practical treatize, 

 named Pantometria, diuided into three bookes, longi- 

 metria, planimetria, and stereometria, &c.," first pub- 



1 Philosophical Ma^izinc, vol. xxviii. (1846), note by de Morcan, 

 pn. 387-q. Poggendorjff's Annalen. vol. cxxxiii. (1868), pp. iga. 349. Zeit- 

 sckri/t/ilr Vemi esuingsmcsen (1880). p. 55 : (i88l>. p. 321 ; (igo8). pp. 81-91 

 and 113-25. Vopler's Prtikti^cltt; Ceomctric (i88t;). p. 361. Proc. Inst. 

 C.E-, vol. clxxiii. (1907-8). p. -139. Preussisclte Jahrbiicher, note by Prof. 

 DIdolff, vo'. cxvi, fTqo4), pp. 362-4. 



NO. 2087, VOL, 81] 



Reproduction of Digges' illustr.'ition of his " Theodelitus." 

 Zeitschri/t/iir Vermcssungs-luescn. 



substituted by others; also from eeaiu (I .see) and 5oAix<is 

 (long). Another suggested derivation is the English 

 article " the " combined with the Arabic " alidhada." 



In searching for a more satisfactory solution, the idea 

 occurred to the writer that the word would naturally be 

 compounded to represent the principal parts of the instru- 

 ment, and when reading Prof. E. Hammer's latest and 

 most interesting discussion in the Zeitschrift fiir Vermess- 

 ungswesen, vol. xxxvii. (1908), pp. 8i-gi and 113-25, he 

 was impressed by one of the illustrations reproduced of 

 Digges' " theodolitus " and description of it, with special 

 mention of the words "sightes," "index," and "double 

 scale." He would submit, therefore, that the true 

 etymology is from the Greek words fltd = a sight; o5EX(fs = 

 any pointed instrument; !'Ti's=a circle or a felloe of a 

 wheel. These Greek words appear to be those which 

 would actually denote the three essential parts of the 

 instrument, viz. the sight, the index arm, or alidade 

 (Digges uses the word index, never alidade), which is re- 

 presented as a pointed instrument, and the limb of 

 graduated circle. The spaces on the circle appear like the 



