NEWTONIAN CONSTANT OF GRAVITATION. 7 
micrometric observation. This, in fact, corresponds, but not exactly, with the second 
case mentioned above, where a gradual change of temperature is going on in the 
surrounding space ; those parts of the apparatus that are massive will lag behind in 
temperature more than the lighter and thinner parts, and, as was pointed out by 
CAVENDISH, this is especially the case in apparatus for measuring the Newtonian 
constant of gravitation. The large lead balls are sure to be hotter or cooler than the 
light rectangular box, and, when hotter, by warming the side of the box near to 
them they set up a circulation, which, in the apparatus of CAVENDISH, produced an 
appearance of attraction. 
Tf it is supposed that after all has acquired a uniform temperature a slight change 
occurs in the surrounding space, then the asymmetrical store of heat will, in the 
case of a large apparatus, be n> times as great as in the other. As before, the 
conductivity will be n times as great, so that an asymmetrical distribution of 
temperature will be 7 times as great, and will Jast 1 times as long in the large as in 
the small apparatus. 
Before I come to describe the apparatus which forms the subject of the present 
paper, I wish to explain why I have employed what may appear objectionable, viz., 
mixed units. I applied to Mr. Cuanny, at the Standards Office, for his opinion, as 
to the limit of accuracy with which he could verify certain lengths and masses. The 
lengths upon which the accuracy of the whole research would depend were to be of 
the order of 1 inch and 6 inches. If I could, as he considered certain, have them 
determined more accurately in relation to the standard 1-inch than I could in 
relation to the centimetre. it would be preferable to have the main dimensions of 
the apparatus set out in terms of the inch, and for construction in England there 
were practical advantages in adopting the inch system. On the other hand, the 
cathetometer that I used (Cambridge Scientific Instrument Co.’s), and the screw 
micrometer (Exiior), both of which were required to make measures of only 
secondary importance, were divided in centimetres. I have therefore had to make 
use of both kinds of measures, but have retained the inch as my standard. With 
respect to the masses, no difficulty could arise in obtaining the necessary accuracy, 
whether pounds or grammes were used. Having gramme weights I was led to 
make all the weighings in grammes, except where, owing to an insufficiency, I 
had to make up with a standard 7 Ib. and 4 lb. weight belonging to the South 
Kensington Museum. These were determined in grammes, and expressed as such. 
Circumstances have therefore compelled me to carry out my experiments on the 
inch gramme second system, and this I have done, finally converting the values 
for G so found into the C.G.S. system, by multiplying by the number of cubic 
centimetres in a cubic inch. (2°53995)? = 16°3861. 
As the suspended masses take up slightly different positions according as they are 
attracted by the large balls in one or the other direction, I was most careful in the 
design to arrange that the apparatus, with the exception of these balls, should be one 
