270 
WALORE 
| JANUARY 21, 1897 
securing very perfect images of the invisible electric discharge 
without the plates being exposed to either X-rays or light. 
This discharge—or possibly, more strictly speaking, the 
electrified streams of air driven off by it—appears to act upon 
It is thus possible to secure 
the plate exactly as light does. 
Fic. 1. 
impressions of such discharges by simple electrification and 
subsequent development. 
Under certain conditions very perfect images of the relief 
upon coins and similar objects can be obtained. This seems to 
Fic. 2. 
account fully forthe fact that in some cases radiographs of coins 
have shown some trace of the design upon the under side which 
was in contact with the film. 
I enclose prints of the radiograph showing the set of the 
silver particles around the skeletons, which effect I have since 
NO. 1421, VOL. 55] 
| reproduced, and also of what I think may properly be called 
electrographs of coins, and of discharges from metallic points 
and surfaces. 
I should be glad to know if any similar results have come 
within the experience of any of your correspondents. 
Fig. 1 is a radiograph of wire skele- 
tons enclosed in cardboard figures, 
developed and fixed, covered with 
glass plate, and put in lantern. On 
the heat of the lantern softening the 
film, the precipitated silver particles 
set themselves in pattern. The ex- 
planation which suggested itself was 
that this was an electrical effect induced 
by the Rontgen tube, but I cannot 
definitely assert that this is the case. 
It is conceivable that: the segregation 
of the particles may be due to some 
other play of forces, such as unequal 
tension in the film; but the first idea 
seems the most probable. I hope to 
test this further by experiment. I have 
reproduced a similar pattern, though 
not quite so perfectly as in this in- 
stance. The irregular edge is the 
result of the partial drying of the film. 
Fig. 2 represents an aluminium 
medal and gold coin. The coins 
were laid upon a photographic dry 
plate, enclosed in a cardboard box, 
electrified for two seconds from one 
pole of a small induction coil, and 
developed. Brush discharge round mar- 
gin very fine, the discharges from the 
two objects repelling each other. The 
larger was in high relief, and the 
lettering has produced small brush 
discharges. Some shaded ground in 
recessed part of coin, probably due to the electrified film of 
air, confined within the margin of the coin, resting in contact 
with the plate. James PANSON. 
Fairfield House, Darlington, January 7. 
The Force of a Pound. 
PRror. PERRY, in his review of my ‘‘ Elements of Mechanics” 
in your issue of November 19, 1896, gives his method of explana- 
tion of mechanical units to engineering students. 
The method is almost as perplexing as the one he so severely 
condemns. The source of confusion in both cases is in at- 
taching the term ‘‘mass” to the ordinary gravitation system 
—the system of ‘*weights and measures.” Engineers have 
no need of the term; in its strict sense it is foreign to 
their work. The engineering unit of quantity is the 
‘*pound,” as determined by the process of weighing against 
standard weights. : 
The engineer deals mainly with bodies at rest, or moving with 
uniform speed. The system sufficient for him is therefore not 
sufficient for the physicist, to whom the idea of acceleration is 
fundamental. The physicist notices that bodies possess a certain 
quality determinative of acceleration, and to this he gives the 
name mass. Masses are thus to be compared by kinetical 
methods, fundamentally at least. The term ‘‘ mass ” belongs to 
the system of the physicist, the so-called absolute system, and to 
it only. 
To sum up. Use the term weight in its legal sense, which is 
that understood by the engineer and by people in general ; define 
mass with reference to acceleration, and not as “‘ quantity of 
matter’; understand that the passage from an absolute to a 
gravitation system [not from mass to weight] is by means of 
a suitable factor with a corresponding change of unit, and all 
confusion vanishes. 
The agitation in favour of an absolute system involving the 
‘* poundal” should be discouraged for the reason, among others, 
that the adoption of the metric system is delayed in consequence. 
The metric system alone is sufficient for both engineer and 
physicist. T. W. WRIGHT. 
Schenectady, N.Y., December 17, 1896 
