February 4, 1892.] 
JOURNAL OF HORTICULTURE AND COTTAGE GARDENER. 
91 
ROYAL METEOROLOGICAL SOCIETY. 
The annual general meeting of this Society was held on Wednesday 
evening, the 27th ult., at the Institution of Civil Engineers, 25, Great 
George Street, S.W. ; Dr. W. Marcet, F.R.S., Vice-President, in the 
chair. The report of the Council for the past year showed the Society 
to be in a very satisfactory position. In May the library and offices 
were removed to more commodious premises at 22, Great George Street. 
After defraying the cost of fitting up the new offices, and the increased 
rental, there still remained a balance in hand of £224. Thirty-four new 
Fellows were elected during the year, the total number on the roll of the 
Society now being 552. 
Owing to the absence of the President, Mr. Baldwin Latham, 
M.Inst.C.E., through an attack of influenza, his address on “Evapora¬ 
tion and Condensation ” w..s read by the Secretary. The question of 
evaporation is as of great importance as the study of the precipitation 
of water on the face of the earth, as the available water supplies of the 
country entirely depend upon the differences between these two sets of 
observations. The earth receives moisture by means of rain, dew, hoar 
frost, and by direct condensation. It loses its moisture very rapidly 
by evaporation. Although evaporation mainly depends upon the differ¬ 
ence between the tensional force of vapour due to the temperature of the 
evaporating surface and the tensional force of the vapour already in the 
atmosphere, yet it is largely influenced by the movement of the air, and 
by its dryness or the difference between the dew point and the actual 
air temperature. Evaporation goes on at night so long as the water 
surface is warmer than the dew point. With sea water the evaporation 
is about 4i per cent, less than with rain w.ater, while with water 
saturated with common salt the evaporation is 15 per cent, less than 
with rain water. 
In his experiments Mr. Latham used an evaporating gauge made of 
copper, 1 foot in diameter, and containing 1 foot in depth of water, 
which was floated by means of a hollow copper ring, placed (i inches 
distant from the body^of the evaporator, and attached to it by four radial 
arms. This form of evaporator was found extremely convenient in 
carrying on all evaporation experiments. It was floated in a tank 
4 feet in diameter, containing 30 inches depth of water. During the 
period of thirteen years, from January, 1879, to December, 1891, this 
evaporator has never once been out of order, or been interfered with in 
the slightest degree by frost. 
Experiments were made with some 6-inch evaporators as to the effect 
of colour on the amount of evaporation,one being painted white, another 
black, and the results given by these gauges were compared with a 
copper gauge exposed under similar conditions. This comparison was 
the means of showing that the gro.rtest errors in evaporating gauges 
arise from the capillarity of the water rising on the sides of the gauge, 
and thus inordinately increasing the amount of evaporation. Con¬ 
sequently a small gauge having a larger amount in proportion of side 
area than a larger gauge, gives a very much greater amount of 
evaporation. 
The results from the floating evaporator, 1 foot in diameter, show 
that the average amount of water evaporated annually during 1879-91 
was 19'948 inches. It was found, however, that as a rule, during the 
period from October to March there were certain occasions when con¬ 
densation was measured. The amount of these condensations in 
thirteen years averaged O'308 inch per annum. Tlie 5-inch evaporating 
gauge freely exposed to atmospherie influences, gave during the same 
perimi (1879-91) an average annual depth of evaporation equal to 
38T85 inches. 
The average annual evaporation, during the three years 1879-81, 
from the 5-inch copper gauge standing in water was 27-90 inches, from 
one painted black 22-97 inches, and from another painted white 21-74 
inches, whilst a gauge of the same dimensions, freely exposed in the 
atmosphere, gave in the same period, 30-90 inches, and the 1-foot float¬ 
ing evaporator, 19-40 inches. The 5-inch copper gauge gave a larger 
amount of evaporation than the gauge painted black. 
Mr. Latham next described some percola'ion experiments which 
wore carried out by Mr. C. Greaves at Old Ford, by Messrs. Dickinson 
and Evans at Ilcmel Hempstead, and by Sir J. B. Lawes and Dr. 
Gilbert at Rothamsted. He then detailed the results of his own ex¬ 
periments, and also the gaugings of the underground waters in the 
drainage areas of the rivers Wandle and Graveney. 
He further stated that in the course of his observations on the flow 
of underground water, he had observed that at certain particular 
seasons of the year it was possible to indicate the direction and volume 
of the flow of underground streams, oven when they were at a con¬ 
siderable depth, owing to the formation of peculiar lines of fog. 
Dr. C. Theodore Williams was elected President for the ensuing year. 
CROSSING PLANTS. 
The term “ cross ” is used to denote the offspring of union between 
plants, whether of different species or varieties, or even different flowers 
upon the same plant. There are different kinds of crosses. One of these 
is the hybrid, or a cross between two species, as a Plum and a Peach, or 
a Raspberry and a Blackberry. Crosses between varieties of one species 
are termed “ half-breeds ” or “cross-breeds,” and those between different 
flowers upon the same jfiant are called “ individual crosses.” Distinct 
species, however, as a rule, refuse to cross. If we apply the pollen of a 
Hubbani Squash to the flower of the common field Pumpkin, the fruit 
will not form. The same is true of the Pear and the Apple, the Oat and 
the Wheat, and most very unlike species. Or the pollen may “ take ” 
and the seeds may grow, but the plants which they produce may be 
wholly barren, sometimes even refusing to produce either flowers or 
seeds, as in the instance of some hybrids between the Wild Goose Plum 
and the Peach. Sometimes the refusal to cross is due to some difference 
in the time of blooming, or some incompatibility in the structure of the 
flowers. But it is enough to know that there are characters in widely 
dissimilar plants which prevent intercrossing, and that these characters 
are just as positive as are size, colour, productiveness, and other characters ; 
that is, the checks to crossing have been developed through the principle 
of universal variability and natural selection, just as other characters 
have been established. The result is simply that the best results of 
crossing are obtained when the cross is made between different indi¬ 
viduals of the same variety, or, at farthest, between different individuals 
of the same species. In other words, hybrids—or crosses between species 
—are rarely useful, and it follows, as a logical result, that the more 
unlike the species the less useful will be the hybrids. 
Again, crossing alone can accomplish little. The chief power in the 
progression of plants appears to be selection. Selection is the force 
which augments, developes, and fixes types. Man must not only 
practise a judicious selection of parents from which the cross is to come, 
but he must constantly select the best from among the crosses, in order 
to maintain a high degree of usefulness and to make any advancement; 
and it sometimes happens that the selection is much more important to 
the cultivator than the crossing. I do not wish to discourage the 
crossing of plants, but I do desire to dispel the illusion which too often 
hangs about it. 
The improvement of existing varieties by crossing is a more important 
office than the summary production of new varieties. This is the chief 
use which Nature makes of crossing—to strengthen the type. Think, 
for instance, of the great rarity of hybrids or pronounced crosses in 
Nature ! No doubt all the authentic cases on record could be entered 
in one or two volumes, but a list of all the individual plants of the 
world could not be compressed into ten thousand volumes. There are a 
few genera, in which the species are not well defined or in which some 
character of inflorescence favours promiscuous crossing, in which hybrids 
are conspicuous; but even here the number of individual hybrids is very 
small in comjiarison to the whole number of individuals; that is, the 
hybrids are rare, while the parents may be common. 
Darw-in w.as the first to show that crossing within the limits of the 
species or variety results in a constant revitalising of the offspring, and 
that this is the particular ultimate function of the operation. Darwin’s 
results are, concisely, these—self-fertilisation tends to weaken the off¬ 
spring ; crossing between different plants of the same variety gives 
stronger and more productive offspring than arises from self fertilisation ; 
crossing between stocks of the same variety grown in different places, or 
under different conditions, gives better offspring than crossing between 
different plants grown in the same place or under similar conditions ; 
and his researches have also show-n that, as a rule, flowers are so con¬ 
structed as to favour cross-fertilisation : in short, he found, as he 
expressed it, that “Nature abhors perpetual self-fertilisation.” Darwin’s 
well-known e-xperiments show that crosses between fresh stock of the 
same variety were nearly 30 per cent, more vigorous than crosses 
between plants grown side by side for some time, and over 44 per cent, 
more vigorous than plants from self-fertilised seeds. On the other hand, 
experiments showed that crosses between different flowers upon the 
same plant gave actually poorer results th.an offspring of self-fertilised 
flowers. It is evident, from all his experiments, that Nature desires 
crosses between plants, and, if possible, between plants grown under 
somewhat different conditions. 
In extended experiments of my own in the crossing of Punapkins, 
Squashes, and Gourds, carried on during several years, increase in pro¬ 
ductiveness due to crossing has been marked in many instances. Marked 
increase in productiveness has been obtained from Tomato crosses, even 
when no other results of crossing could bo seen. 
Almost every farmer and gardener at the present day feels that an 
occasional change of seed results in better crops. Much of the rapid 
improvement in fruits ami vegetables in recent years is due to the 
practice of buying plants and seeds so largely of dealers, by means of 
which the stock is often changed. Even a slight change, as between 
farms or neighbouring villages, sometimes produces more vigorous plants 
and often more fruitful ones. In the cross, a new combination of 
characters or a new power fits it to live better than its parents in the 
conditions under which they lived. In the case of change of stock we 
find just the reverse, which, however, amounts to the same thing, that 
the new characters or powers fit the plant to live better in conditions 
new to it than plants which have long lived in those conditions. In 
cither case the good comes from the fitting together of new characters or 
powers and new environments. 
Crossing within the variety and change of stock within ordinary 
bounds arc therefore beneficial, and the results in the two cases seem to 
flow from essentially the same causes, and crossing and change of stock 
combined give much better results than either one alone. These pro¬ 
cesses are much more important than any mere groping after new 
varieties, not only because they are surer, but because they are universal 
and necessary means of maintaining and improving both wild and 
cultivated plants. Upon this point Darwin expressed himself as 
follows :—“ It is a common practice with horticulturists to obtain 
seeds from another place having a very different soil, so as to avoid 
raising plants for a long succession of generations under the same 
conditions ; but with all the species which freely intercross by the aid 
