Notices respecting New Books, 459 



slightly lengthened and shortened, and that the weight of the truss 

 can be neglected. If the beam were not trussed, the deflection at 

 the middle point would be, according to the timber employed, 

 between a half and a quarter of an inch, and at E and E slightly 

 less. If, then, we suppose A B and C D lengthened sufficiently 

 to allow of the points E and E drooping to this extent, the com- 

 pressive stress along A D instead of being 24 tons, will be exactly 

 nothing, except so far as the upper part of the beam is compressed 

 in the act of bending. If, now, A B and CD are gradually shortened, 

 the component members of the truss will be brought into a state 

 of stress, the amount of the stress being determined by the degree 

 in which the droop at E and E is reduced. At each turn of the 

 nuts the stresses will be varied ; and when the operation is com- 

 pleted, the amounts of these stresses will in themselves be determi- 

 nate, but will depend upon the extent to which the process of 

 screwing up has been carried. If two such trusses were placed side 

 by side, under apparently similar circumstances, it is quite possible 

 that their actual states of stress might be materially different. 



In spite of these defects, and some others which our limits will 

 not allow us to notice, the book contains very much that is valuable, 

 and we can cordially recommend it to the notice of students of 

 mechanical science, 



British Rainfall, 1872. Compiled hy Gr. J. Symons. London : 



E. Stanford, Charing Cross. 

 W The well-earned character of this very useful annual volume is 

 fully maintained in the present issue, and not only so, but it con- 

 tains three features of unusual interest — an important article on 

 the measurement of snow, an examination of the alleged periodi- 

 city of rainfall synchronous with the period of sun-spot frequency, 

 and a comparison of rainfall in 1872 with the three years of great 

 rainfall, 1848, 1852, and 1860. In the article on the periodicity of 

 rainfall, Mr. Symons enters at some length into the researches of 

 Messrs. Meldrum and Lockyer, and quotes an elaborate paper by 

 Dr. Jelinek to the effect that taking a broad view of the sub- 

 ject, the evidence is conflicting — a result that Mr. Symons had pre- 

 viously arrived at. Dr. Jelinek urges the importance of obtaining 

 still further materials for estimating the connexion between sun- 

 spot frequency and depth of rain, and advises that the considera- 

 tion of the data should be proceeded with quietly and without pre- 

 judice. At the close of the article Mr. Symons gives a list of some 

 links in the chain which binds us to our central source of light and 

 heat, in which we find " aurorse accordant with sun-spot prevalence." 

 This reminds us that there is another link not mentioned by Mr. 

 Symons, viz. the diurnal inequality of magnetic declination. Pro- 

 fessor Loomis has furnished the 'American Journal of Science and 

 Arts ' with an interesting paper on the synchronous variations of 

 the three phenomena — sun-spot frequency, mean diurnal range of 

 magnetic declination, and auroral displays, from 1776 to 1872, 

 illustrated with the curves of each. Erom the third feature above 



