66 



EEPORTS ON THE STATE OF SCIENCE. — 1915. 



but will serve to give an idea of the magnitude of the quantities 

 involved. 



It will be seen that Eskdalemuir, Helwan, and Malta show no lunar 

 tides ; Seychelles and Cocos have large tides, as well as large diurnal 

 effects. 



It may be presumed that 0-001 in. = 0"'01 approximately, but in 

 most cases no more precise scale value can be recovered from the records. 



IX. Insect Disturbances of Seismogra'phs. 



An inquiry from St. Helena suggests that it may be useful to other 

 observers to "print a note on the disturbances caused by insects. 



Mr. J. J. Shaw has kindly drawn up the following : 



A difficulty which is frequently met with in practical seismology 

 is to keep the seismograph free from the various insect interferences. 

 An imprisoned moth or fly will often keep the boom in a state of unrest 

 for several days at a time ; but a much more serious nuisance is the 

 ubiquitous spider ; he not only makes havoc with the trace, but also 

 ties up the boom, and very greatly destroys the sensitivity of the apparatus. 

 It is useful to be able to decide from the trace whether the trouble is 

 of the first or second order ; because if of the second it is not sufficient 

 to get rid of the spider, but the web must also be removed. 



There is an advantage in making an artificial disturbance each day 

 by standing for about 15 seconds on a selected spot near the side of the 

 masonry column; a suitable time is just before changing the film, as 

 the boom is then at rest. This will give a standard deflection which 

 can be compared day by day whereby any loss in efficiency is quickly 

 detected. The decrement curve at the commencement of the film is 

 also useful in identifying the cause of these troubles. In instances of 

 the first type the prisoner periodically sets the boom in motion, and 

 occasionally leaves it to come to rest, when the trace wiU be seen to con- 

 tinue in alignment with its previous position. Confirmation may be 

 looked for in the unimpaired efficiency indicated in the decrement curve. 

 If, however, the disturbances produce permanent displacements in the 

 trace they are probably caused by a spider, or perhaps a moth, whose 



