This study attempts to evaluate the field performance of various mining equipment used at the development galleries of coal mines. These are hand-held and jumbo rock drills, and a roadheader used in mechanical excavation. For this purpose, the penetration rates of rock drills were monitored and measured in the field. The physical, mechanical, and drillability properties were determined through the collected samples in order to understand the complex interactions between the rock and bit/pick. The abrasive mineral content was also analyzed with XRD analysis to examine the wear on the cutting/drilling tools. Besides, the specific energy of the equipment was calculated relying on the operational parameters. A comparison of the monthly advance and production rates of the drilling rigs and roadheader was made. The relations among operating power, specific energy, and design of buttons/picks were investigated. It has been found that the average advance and production rates of the mining equipment are consistent with the penetration rate. The results verified that the roadheader used in mechanical excavation and the jumbo drill used in drilling and blasting technique are the machines maximizing the advance and production rates.
Between 2014 and 2016 the number of foreigners on the Polish labour market increased by over 300 per cent. They were mainly Ukrainian citizens taking up seasonal employment on the basis of the so-called ‘sim-plified system’. According to the literature, such a large increase in labour immigration in a short period of time may be an important factor in the growth of unemployment and the reduction of the employment rate of natives. The main purpose of this text is to show the correlation between the increase in the employment of foreigners in Poland and to determine whether or not this has had an impact on the deterioration of the state of the labour market. For this purpose, data from the Central Statistical Office and the Polish Ministry of Family, Labour and Social Policy were collected. As a result of our analysis, it was found that the significant increase in the presence of foreigners on the Polish labour market, which the country has faced in recent years, was not correlated with the rise in unemployment, the increase in the rate of economic inactivity and the availability of seasonal jobs. On this basis, we can state that the increase in the supply of foreigners on the Polish labour market, compared to other factors influencing it, was weak enough for the negative effects of the increased employment of foreigners to not occur.
Luminescence dating is based mainly on the dosimetric properties of quartz and feldspar. These minerals are among the most popular found on Earth, resulting in the possibility of using luminescence methods in practically any environment. Currently, quartz remains the best recognized mineral in terms of dosimetric properties, particularly with regards to results obtained for quartz grains, which are regarded as being the most reliable in luminescence dating. Supporters of luminescence methods are constantly growing, however, these groups do not always have sufficient knowledge to avoid even the most basic of issues that may be encountered overall – from the process of sampling through to the awareness of what a single luminescence result represents. The present paper provides an overview of several practical aspects of luminescence dating such as correct sampling procedures and all necessary information regarding the calculation of the dose rate and equivalent dose with particular reference to potential problems that occur when the age of the sample is being determined. All these aspects are crucial for obtaining a reliable dating result, on the other hand, they remain a potential source of uncertainty.
In the summer of 1977/78, materials for determining the age and growth rate of Pseudochaenichthys georgianus Norman, 1937 were obtained in the South Georgia region. The 11—51 cm long fishes were of the age 1+ to 13+. The curvilinear relationship between the body length and the otolith radius took an S shape. In the first four years annual increments are almost identical and start to decrease in the following years. Spawning may occur in the 5th or 6th year of life, and hatching of larvae from eggs laid in autumn (IV—VI) takes place at the end of winter (IX of X). Larvae and juvenile fish lead an exclusively pelagic life, while older fish feed in near-bottom layers: when resources are scare there they seek food in the pelagial.
Several conclusions and recommendations concerning sediment trap geometry, the technique of their deployment and interpretation of measurements results are described in this paper. Only cylindrical sediment traps are able to cope with the diverse and dynamic environment of glaciated fjords. The relation between different trap parameters shows the optimal proportion of cylinder diameter as being between 6 and 10 cm and ratio length/diameter not less than 7/1. During the peak of the melting season in Kongsfjorden (Spitsbergen) the rate of sedimentation of total matter reaches over 900 g m2 d1 and the velocity of brackish water current can reach 80 cm s1 on the surface. Owing to the high productivity of Arctic fiords and large concentration of suspended mineral matter it is possible to collect of large samples in a short time, therefore prevention of sediment traps by swimmers is not necessary.
The article considers the issues of the value of invested capital, methods of its measurement and its growth mechanisms. The author draws attention to relationship between the value of capital and the paradigm of economics, which ultimately indicates the existence of connections between the effectiveness of investment and the philosophy of economics. The main purpose of the article is to identify abnormalities in the valuation of assets by investors due to their incorrect or incomplete understanding of the value growth mechanism, the effects of which may assume significance on a macroeconomic scale.
The paper deals with the issue of financial efficiency, measured by the arithmetic rate of return, of indirect financial investments in the area of strategic raw materials (hard coal, copper, crude oil). Two forms of indirect investments were analyzed: shares of natural resources companies listed on the Warsaw Stock Exchange and futures contracts for strategic commodities: hard coal, copper and crude oil.
The time of the analysis is the first 6 months of 2019 and 2020. The year 2019 was regarded as an analysis of the period of economic growth, and the year 2020 was the analysis of the period of economic crisis. The comparisons were made in two dimensions. Firstly, it whether indirect commodity investments show the characteristics of efficiency resilience to the time of the economic crisis was checked (by comparing the achieved rates of return in the two analyzed periods). Secondly, which of the analyzed forms of investment (stocks, contracts) gives better investment results during economic growth and economic crisis was compared.
As it was shown in the paper, indirect commodity investments do not show an above-average rate of return neither during economic growth nor economic crisis. The achieved rates of return on shares compared to changes in the WIG20 index in the analyzed first half of 2019 were negative. Only one company showed a positive and significantly higher than the market rate of return. Very similar results were achieved by the analyzed companies in 2020.
On the other hand, the analysis of prices and rates of return on commodity futures contracts showed that in the period of economic growth it is effective to take a long position on crude oil contracts and a short position on hard coal contracts. In a period of economic crisis, the opposite position is profitable due to the observed growth in hard coal prices and a significant drop in crude oil prices.
The answers to the research questions posed in the paper do not provide indications for recommending indirect forms of investment in commodities as an alternative to analogous forms of other sectors of the economy. The analysis shows that the impact of the economic situation on the efficiency of commodity investment is most noticeable for crude oil, and the least (among the analyzed commodities) for indirect copper-based investments.
This study included investigation of efficiency of the threshold used to classify symptoms as present, investigation of efficiency of the cut-off point used to identify potentially addicted to work individuals, investigation of magnitude of the problem of class overlap, and investigation of effects of dichotomization of polytomous items on the estimates of the latent trait level. The sample comprised 16,426 working Norwegians (Mage = 37.31; SD = 11.36) who filled out the Bergen Work Addiction Scale (BWAS). The results showed that the difficulty/third threshold parameters corresponding to the threshold used to classify symptoms as present were lower than 1.5 for the items corresponding to tolerance and conflict and higher than or equal to 1.5 for the items corresponding to salience, mood modification, relapse, withdrawal, and problems. The cut-off point used to identify individuals as potentially addicted to work identified 411 individuals (31.9% of all individuals classified by the polythetic approach as potentially addicted to work) whose estimates of the latent trait level were lower than 1.5 as potentially addicted to work. The problem of class overlap (being classified by the polythetic approach into different class despite almost the same level of the latent trait) affected 4,686 individuals (28.5% of the whole sample). The dichotomization of polytomous items had a substantial effect on the estimates of the latent trait level. The findings show that the polythetic approach is not efficient in identifying potentially addicted to work individuals and that the prevalence rates of work addiction based on the polythetic approach are not trustworthy.
Biological parameters of the larval parasitoid Cephalonomia tarsalis (Ashmead) (Hymenoptera : Bethylidae) and its host the saw-toothed beetle Oryzapehilus surinamensis (L.) (Coleoptera : Silvanidae) were studied in the laboratory. The duration of the immature period, survival during development, as well as adult longevity and the number of progeny of both insects were recorded. Our data were used for the estimation of several demographic parameters and life table construction of both the host and the parasitoid. The wasp managed to complete its development (egg – adult) in 19.8 days at 25oC, whereas the adult female lived for 24.3 days. The host O. surinamensis demonstrated a longer developmental period (30.5 days) and adult female longevity (103.0 days). Female wasps laid an average of 66.4 eggs throughout their lifetime whereas their beetle hosts laid five times more eggs (313.9). Life table parameters of C. tarsalis were estimated for the first time. The intrinsic rate of natural increase (rm) was 0.124 which was almost double that of its host (0.056). Our results are discussed on the basis of evaluating and improving the performance of C. tarsalis as a biocontrol agent against O. surinamensis in storage facilities.