534 



REPORT 1886. 



end of tlie day the clock shows the number of hours and minutes of sunshine for 

 the day. A note is then made of the recorded total, and the hands put back to zero 

 again. 



The drum is covered with ' cyclostyle ' paper, and after the three months' record 

 is complete it is removed and put in a frame, under which can be placed ordinary 

 white paper. Printer's ink is then rolled over it, and a great many copies of the 

 original record can thus be printed for distribution. I think it might be of interest 

 to print several yearly copies on the top of each other on the same paper. By the 

 general resulting degrees of light and shade we should be able to see if there were, 

 any periods in the year which were liable to more sunshine than others. 



13. Second Bejporf, of the Commiitee for promoting Tidal Observations 

 in Canada. — See Reports, p. 150. 



14. Report of the Committee for inviting designs for a good Differential 

 Gravity Meter. — See Reports, p. 141. 



15. Description of a Differential Gravity Meter founded on the Flexure of 

 a Spring. By Sir W. Thomson, LL.D., F.B.8. 



The design and construction of the instrument now to be described was under- 

 taken on the suggestion of General Walker, of the East Indian Trigonometrical 

 Survey. At the Aberdeen Meeting of the British Association in 1885, General 

 Walker obtained the appointment of a committee to examine into the whole 

 question of the present methods and instruments for the measurement of gravi- 

 tational force, and to promote investigation, having for its object the production of 

 gravitation-measuring instruments of a more reliable and accurate character than, 

 those now in use. 



The secretary of this committee. Professor Poynting, has already issued a 

 circular note to the members of the committee (of whom the author is one), stating 

 the conditions which must be fulfilled by any gravimeter laid before the committee 

 for examination and report. 



An instrument, constructed according to the following description, promises to 

 fulfil all the conditions mentioned in Professor Poynting's circular. Its sensibility 

 is amply up to the specified degree. It is, of necessity, largely influenced by 

 temperature, and it is not certain that the allowance for temperature, or the means 

 which may be worked out for bringing the instrument always to one temperature, 

 will prove satisfactory. It is almost certain, although not quite certain, that the 

 constancy of the latent zero of the spring will be sufficient, after the instrument has 

 been kept for several weeks or months under the approximately constant stresa 

 under which it is to act in regular use. 



The instrument, which is represented in the accompanying sketch, consists of 

 a thin flat plate of springy german silver of the kuid known as ' doctor,' used 



