142 REPORT — 1886. 



The instrument did not give satisfactory results, and Sir C. W. Siemens 

 was led to devise another form in which the weight of a column of 

 mercury was supported by two spiral steel springs. If gravity increased 

 the weight increased and the springs were stretched. An increase in 

 their length was observed by a micrometer screw, the moment of contact 

 being given by an electric signal. This instrument gave much better 

 results than the first, but it would require much improvement before it 

 could be brought into use for differential measures of gravity. 



The Committee will be glad to receive suggestions from any who are 

 interested in the subject, and any design submitted to them will receive 

 careful attention. 



The following conditions should be satisfied by the instrument : — 



It should be portable. 



It should be capable of use in ordinary buildings and under varying 

 conditions of temperature and pressure. 



Effects of change of temperature should be ascertainable, so that they 

 may be allowed for. 



The zero point should remain fixed if the temperature and gravity are 



the same. 



It should not be affected by terrestrial magnetism. 



It should give variations of ^^0^,^00 in the value of gravity. 



Sir Wm. Thomson has favoured the Committee with the following 

 account of a gravimeter, designed by himself, for circulation : — 



Spring Gravimeter. 



The following instrument promises to fulfil all the conditions men- 

 tioned in the preceding 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, may prove satisfactory. It is almost certain, although not 

 quite certain, that the constancy of the virtual zero of the spring will be 

 sufficient, after the instrument has been kept for several weeks or months 

 under the approximately constant stress under which it is to act in 

 regular use. 



The instrument consists of a thin flat plate of springy german silver of 

 the kind known as ' doctor,' used for scraping the coloiir off the copper 

 rollers in calico printing. The piece used was 75 centimetres long, and 

 was cut to a breadth of about 2 centimetres. A brass weight of about 

 200 orammes was securely soldered to one end of it, and the spring was 

 bent like the spring of a hanging bell, to such a shape that when held 

 firmly by one end the spring stood out approximately in a straight line, 

 havino- the weight at the other end. If the spring had no weight the 

 curvature, when fi-ee from stress, must be in simple proportion to the 

 distance along the curve from the end at which the weight is attached, 

 in order that when held by one end it may be straightened by the weight 

 fixed at the other end. 



The weight is about 2 per cent, heavier than that which would keep 

 the spring straight when horizontal; and the fixed end of it is so held 

 that the spring stands not horizontal but inclined at a slope of about 1 

 in 5, vnth the weighted end above the level of the fixed end. In this 

 position the equilibrium is very nearly unstable. A definite sighted 



